John was with someone at the time he ordered his first beer. The beer he ordered wasn't for himself, it was for the person with him who somehow couldn't communicate with the lady. Therefore, the lady asked John for an ID, which wasn't John's.

The next time John ordered a beer, he was alone. The lady refuses asks for his specific ID, and refuses to serve him beer

Another possibility is that the lady who asked for John's ID for his first beer, wasn't a waiter at all, it was just some random lady who thought it was OK for John to drink under aged. The second lady whom John asked for a beer, was in fact a waiter in service, being subjected to drinking laws, and therefore refused to serve John a beer.

Another possibility is that the second waiter wasn't the same waiter as when John first ordered a beer. The second waiter is under aged as well and it is therefore illegal for her to serve John a beer.

And yet another possibility is that the person who asks John for his ID four hours later, is actually the police who are looking for John as a suspect in some crime. The waiter glances at the ID, identifies that the suspected criminal is in fact this specific John Johnson, and informs John that it is illegal for her to serve him a beer while he is being arrested.