Why canít I place my puzzle pieces on the puzzle I want to finish?

I get three pieces in the last derby and ALL three of them get placed on a puzzle that isnít even close to completion while the puzzle Iím close to completing doesnít get a single piece. This is so frustrating. I should be able to pick the puzzle I want to place them. This happens all the time and itís especially frustrating when I only need one piece left and they all go to a different puzzle.