Why am I not receiving some of the LEGO Harry Potter: Years 1-4 trophies?

Unfortunately, there are bugs with the co-op mode in LEGO Harry Potter: Years 1-4, which can result in any of the "buy all types of characters" trophies not rewarding properly. I hate to say this, but the only solution is to restart the game from a fresh save and ensure only one player purchases the characters.

From PS3Trophies.org:

Q: Help! My Wonderful Weasly/Quidditch Team/Dark Wizards/Animagus Trophy didnt unlock! Why? What can I do?

A: These trophies are known to glitch if any of the characters needed for the trophy are purchased by the player 2 in co-op mode. There is no known remedy for the situation so far aside from starting the game over for that trophy. The best way to ensure these trophies unlock normally is to make sure player 1 purchases them all. Sometimes it is even better to be safe and have player 2 (if playing Co-op) drop out while you purchase characters.