A lot of people have had this issue
1st - Have you used the cheat app to max everything, if you have unfortunately that could be the cause because cheating to the extend can cause bugs.
As explained here -
You must be registered to see the links
2nd - what version of the game are you playing, latest is 0.8.0c
3rd - if you are playing 0.8.0c then try by using the cheat app on the phone to lower Amy's love to 9 and repeat the drinking scene with her.
4th - if that doesn't work, check Amy's pictures in the Gallery App on the phone if you don't see pictures 2-8 but instead see a padlock, it means that the dream scene with Amy and veronica didn't trigger and I've come to believe that is important to the "give a drink to Amy" scene.
5th - if all else fails try giving this a go -
You must be registered to see the links
6th - Possibe fix by
@Sycho -
You must be registered to see the links