Why can he do the spell right? He can do the Reversal spell right but not the spell they're learning today. What's wrong with him? He should give it another try.

He made a Z shaped movement with his wand. This time he already made sure to picture the kind of cloth he wants for his mannequin very clearly in his head. "Summus Vestis!" The garbage bag slowly turned into a white shirt like what he wanted but the shirt only have one sleeve. The other one was missing.

Second attempt = Fail!

"Reverto!" And the one sleeve shirt turned back into rubbish bag.