float buffXAccToSend[20];
 [...]
 collect()
 {
  if (storeAccData)
  {
    // Filling Buff
    buffXAccToSend[contBuffAcc-1] = accX;
    contBuffAcc++;
  }

  // After 60 values call sendBuffAcc()
  if (contBuffAcc%(sizeBuffAcc+1) == 0)
  {
    sendBuffAcc();
    accDataReady=true;
  }
 }
 [...]

void sendBuffAcc()
{
  int stepCont = 1;

  // Data are store in a structure and then sent
  MyShortCommand * pMyCmdShort = (MyShortCommand *)(&bufferAcc[sizeof(MyControlHdr)]);

  for (int i=0;i<sizeBuffAcc;i++)
  {
    numFilterValuesToSend++;

    pMyCmdShort->cmd = accValuesID;
    pMyCmdShort->param1 = buffXAccToSend[i];

    // You can find the output of this loop at the end of the question
    if (printAckCommands)
    {
      Serial.print(" CMD number: ");
      Serial.println(stepCont);
      Serial.println(pMyCmdShort->param1);
      Serial.println();
    }
    pMyCmdShort++;

    // checks whether all values have been added to the message or the
    // content has reached the maximum dimension 'maxLengthMess'
    if (stepCont%(maxLengthMess) == 0 || stepCont == sizeBuffAcc)
    {
      sendCommandToMatlabFilter();
    }
  }
  stepCont++;
 }
 storeAccData = false;
 contBuffAcc=1;
}

val: 20    -0.00
val: 21    0.02
val: 22    0.02
val: 23    0.01
val: 24    -0.02
val: 25    -0.02
val: 26    0.00
val: 27    0.03
val: 28    -0.01
val: 29    -0.00
val: 30    0.01
val: 31    -0.00
val: 32    -0.01
val: 33    0.00
val: 34    0.01
val: 35    0.02
val: 36    0.02
val: 37    -0.00
val: 38    -0.00
val: 39    0.00
val: 40    0.00

Sending # commands:20
 CMD number: 21
-0.00

 CMD number: 22
0.02

 CMD number: 23
0.02

 CMD number: 24
0.01

 CMD number: 25
-0.02

 CMD number: 26
-0.02

 CMD number: 27  // From now on values are all zero! Why?
0.00

 CMD number: 28
0.00

 CMD number: 29
0.00

[...]

 CMD number: 59
0.00