Finding the last piece of Collection takes too long!

The game is using the hard but fair random system. Everyone gets equal chances of finding collectible items, however the chances decrease based on two factors:
1. The more items you find, the harder it becomes to find more.
2. The longer you play in the same room (trying to complete a collection for example), the lower are the chances to find that last collectible you need.

Try switching rooms from time to time! That might give you just enough of a chance to get the item when you come back.