Anyone else have the occasional rewards issue? (USA)

Occasionally when someone gives me the four digit number to redeem a reward, the POS gives the flag that the code is invalid, and then when I try again it hits me with "this code has already been used." and then the customer is upset because they'll try to use a different one, but have to wait 15 minutes. Every time I just apologize and say I can't do anything about it, but wasn't sure if there was anything I could actually do.