Pong With Primitives and Collision Response Issues

// Pong Code
#include <iostream>
#include <allegro5/allegro.h>
#include <allegro5/allegro_image.h>
#include <allegro5/allegro_primitives.h>
#include <allegro5/allegro_font.h>

using namespace std;

const int SCREEN_W = 640;
const int SCREEN_H = 480;
const float FPS = 60.0;
const int START_X = SCREEN_W / 2;
const int START_Y = SCREEN_H / 2;
enum MYKEYS {
    KEY_W, KEY_S, KEY_UP, KEY_DOWN, KEY_M
};

int main()
{
    ALLEGRO_DISPLAY *display = NULL;
    ALLEGRO_TIMER *timer = NULL;
    ALLEGRO_EVENT_QUEUE *event_queue = NULL;
    ALLEGRO_FONT *font = NULL;
    ALLEGRO_BITMAP *paddle1, *paddle2, *ball, *scrnshot;

    bool key[5] = { false, false, false, false, false };
    bool redraw = true;
    bool doexit = false;

    const int BSIZE = 32;
    int padW = 10, padH = 120;
    int pad1X = 10, pad1Y = SCREEN_H / 2 ;
    int pad2X = SCREEN_W -(10 + padW), pad2Y = SCREEN_H / 2;
    int ballX = START_X;
    int ballY = START_Y;
    int xVel = 4, yVel = 4;
    int p1Score = 0, p2Score = 0;

    int fps = 0, fps_accum = 0;
    double fps_time = 0;

    if(!al_init())
    {
        cout << "Failed to initialize allegro library.\n";
        return -1;
    }

    if(!al_install_keyboard())
    {
        cout << "Failed to install keyboard.\n";
        return -1;
    }

    if(!al_init_primitives_addon())
    {
        cout << "Failed to initialize primitives addon.\n";
        return -1;
    }

    al_init_image_addon();
    al_init_font_addon();

    timer = al_create_timer(1.0 / FPS);
    if(!timer)
    {
        cout << "Failed to create timer.\n";
        return -1;
    }

    display = al_create_display(SCREEN_W, SCREEN_H);
    if(!display)
    {
        cout << "Failed to create display.\n";
        return -1;
    }

    font = al_load_font("data/fixed_font.tga", 0, 0);
    if(!font)
    {
        cout << "Failed to load font.\n";
        return -1;
    }

    event_queue = al_create_event_queue();
    if(!event_queue)
    {
        cout << "Failed to create event queue.\n";
        return -1;
    }

    paddle1 = al_create_bitmap(padW, padH);
    al_set_target_bitmap(paddle1);
    al_clear_to_color(al_map_rgb(0, 191, 255));
    al_set_target_bitmap(al_get_backbuffer(display));

    paddle2 = al_create_bitmap(padW, padH);
    al_set_target_bitmap(paddle2);
    al_clear_to_color(al_map_rgb(0, 191, 255));
    al_set_target_bitmap(al_get_backbuffer(display));

    ball = al_create_bitmap(BSIZE, BSIZE);
    al_set_target_bitmap(ball);
    al_clear_to_color(al_map_rgb(255, 0, 255));
    al_set_target_bitmap(al_get_backbuffer(display));

    al_register_event_source(event_queue, al_get_display_event_source(display));
    al_register_event_source(event_queue, al_get_timer_event_source(timer));
    al_register_event_source(event_queue, al_get_keyboard_event_source());

    al_clear_to_color(al_map_rgb(0, 0, 0));

    al_flip_display();

    al_start_timer(timer);

    while(!doexit)
    {
        ALLEGRO_EVENT ev;
        al_wait_for_event(event_queue, &ev);

        if(ev.type == ALLEGRO_EVENT_TIMER)
        {
            // logic goes here
            if(ballX < 0 || ballX > SCREEN_W - BSIZE)
            {
                if(ballX < 0)p2Score++;
                if(ballX > SCREEN_W - BSIZE)p1Score++;
                ballX = START_X;
                ballY = START_Y;
                xVel = -xVel;
                yVel = -yVel;
            }

            if(ballY < 0 || ballY > SCREEN_H - BSIZE)
                yVel = -yVel;

            ballX += xVel;
            ballY += yVel;

            // collision detection
            if(pad1X + padW > ballX && pad1Y < ballY + BSIZE && pad1Y + padH > ballY)
            {
                xVel = -xVel;
            }

            if(pad2X < ballX + BSIZE && pad2Y < ballY + BSIZE && pad2Y + padH > ballY)
            {
                xVel = -xVel;
            }

            if(key[KEY_W] && pad1Y >= 0)
                pad1Y -= 5;
            if(key[KEY_S] && pad1Y <= SCREEN_H - padH)
                pad1Y += 5;

            if(key[KEY_UP] && pad2Y >= 0)
                pad2Y -= 5;
            if(key[KEY_DOWN] && pad2Y <= SCREEN_H - padH)
                pad2Y += 5;
            if(key[KEY_M])
            {
                scrnshot = al_create_sub_bitmap(al_get_backbuffer(display), 0, 0, SCREEN_W, SCREEN_H);
                al_save_bitmap("data/screens/scrnshot.png", scrnshot);
            }
            redraw = true;
        }
        else if(ev.type == ALLEGRO_EVENT_DISPLAY_CLOSE)
        {
            break;
        }
        else if(ev.type == ALLEGRO_EVENT_KEY_DOWN)
        {
            switch(ev.keyboard.keycode)
            {
                case ALLEGRO_KEY_W:
                    key[KEY_W] = true;
                    break;
                case ALLEGRO_KEY_S:
                    key[KEY_S] = true;
                    break;
                case ALLEGRO_KEY_UP:
                    key[KEY_UP] = true;
                    break;
                case ALLEGRO_KEY_DOWN:
                    key[KEY_DOWN] = true;
                    break;
                case ALLEGRO_KEY_M:
                    key[KEY_M] = true;
                    break;
            }
        }
        else if(ev.type == ALLEGRO_EVENT_KEY_UP)
        {
            switch(ev.keyboard.keycode)
            {
                case ALLEGRO_KEY_W:
                    key[KEY_W] = false;
                    break;
                case ALLEGRO_KEY_S:
                    key[KEY_S] = false;
                    break;
                case ALLEGRO_KEY_UP:
                    key[KEY_UP] = false;
                    break;
                case ALLEGRO_KEY_DOWN:
                    key[KEY_DOWN] = false;
                    break;
                case ALLEGRO_KEY_M:
                    key[KEY_M] = false;
                    break;
                case ALLEGRO_KEY_ESCAPE:
                    doexit = true;
                    break;
            }
        }

        if(redraw && al_is_event_queue_empty(event_queue))
        {
            // draw code goes here
            redraw = false;
            al_clear_to_color(al_map_rgb(0, 0, 0));
            double t = al_get_time();

            al_draw_bitmap(ball, ballX, ballY, 0);
            al_draw_bitmap(paddle1, pad1X, pad1Y, 0);
            al_draw_bitmap(paddle2, pad2X, pad2Y, 0);
            al_draw_filled_rounded_rectangle(4, 4, 100, 30, 8, 8, al_map_rgba(0, 0, 0, 200));
            al_draw_textf(font, al_map_rgb(255, 255, 255), 8, 8, 0, "FPS: %d", fps);
            al_draw_textf(font, al_map_rgb(255, 255, 255), SCREEN_W / 2, 10, ALLEGRO_ALIGN_CENTRE, "%d           SCORE            %d", p1Score, p2Score);

            al_flip_display();

            fps_accum++;
            if(t - fps_time >= 1)
            {
                fps = fps_accum;
                fps_accum = 0;
                fps_time = t;
            }
        }
    }

    return 0;

}