Currently having an issue on one of my forums with the credits not accumulating. Basically they user will "earn" the credit, but the balnce will not increase. It will just overwrite the previous balance and enter whatever the current earning was.
I had set it so the currency can't go in the negative (To prevent people from buying stuff without actually having the currency) and that's when this started to occur. After I put it back to "Allow negative display" the currency started to accumulate again.