Amex periodically FUBAR's the reward calculation for my Costco AMEX. The algorithim is complex enough that I have to do it in a spreadsheet. I set up the spreadsheet as soon as I realized that the reward on this month's statement was calculated from Last Month's spend. Anything that unnecessarly complex could hide a mess o' chiseling.
Sure 'nuff, they started under-reporting the rewards right off the bat.
AMEX restores the reward as soon as I call. I have asked them to set the b@stard flag on my account. B@stard means I balance my accounts to the penny, and I am on the horn if there is the tiniest discrepancy. I believe this minimizes the number of "errors" on my statement. I put errors in quotes, because none have been in my favor. This is statistically unlikely. So under-calculation of rewards has dropped in frequency from every other statement to once a year. (Just to see if I still track?)