Key fob completes program setup...then doesn't work

We have two key fobs for our MZ5. We have one that we use regularly and the other one is the backup. I have programmed the backup one three times in past few months, and it somehow doesn't work after a few days. I followed the MZ5 programming procedures and both fobs work upon completion. Both fobs will lock and unlock, but it's always our backup one that doesn't lock or unlock the MZ5 after a few days.

Am I doing something wrong? Anyone else experience this? If it's a bad fob, I'm curious as to why it works after it's programmed (uhm)(dunno)(shrug)