Terribly sorry for the inconvenience, but I am taking note of this problem and forwarding it to the developers for you. I do recommend you send a ticket to customer service so that you also have a record of it!
Update: I just heard back from the developers and found that although it is a very rare case, there are times when you reach 20/20 and it does not upgrade. The reason is because "WISH" is something that ac*ulates if the item does not succeed in upgrading. Your next upgrade will be 100% successful because your "WISH" is completely full!