You have four Piggy Bank Slots. If all the slots are full, you will not receive the Piggy Bank reward for winning a game. 

In this case, you will need to free up at least one slot by opening a Piggy Bank. Unfortunately, any Piggy Banks that were unable to be delivered will not be restored at a later date and are permanently gone.

Be sure to play games when you have empty Piggy Bank Slots!