Chest not opening (Psychonauts 2)

Issue:

I can't open a chest, even though I definitely have a key! (Short answer - your psy-wallet is probably too full. Buy stuff!)

Solution:

Have you tried really putting your back into it? Nah, just kidding. This one's easy: it can happen if your Psy-wallet is full. Try buying something from Otto and then that chest should be able to open!