Obviously this game isn't super difficult but the problem is that Lego Harry Potter is extremely prone to game-breaking glitches that seem to get more frequent the closer players get to completion.
In my case, I was at 99% and decided to save/close and backup to cloud just as I was about to finish last few trophies, only to discover that both of my saves were corrupted.
This is the X factor for getting your platinum, unexpected and unexplained glitches.
Always assume that this will happen to you and stay one step ahead of glitches by making many back-ups AND for good measure always quit the game from the pause menu before shutting down or closing the application in the PS menu.
Good luck.