You may experience a discount not applying on a single grade/pump - the most often encountered scenario being a customer pressing "yes" on the discount prompt, but the price not dropping below the regular level. That's usually caused by Midax software losing track of the current pricing levels.
Without the relevant current pricing info, the software cannot determine the proper discount.
There are 2 possible solutions to have the prices re-sent to the Midax software - doing any of these two steps should re-apply the correct prices and resolve the problem:
1) Perform a full fuel reinitialization from the Ruby
2) Do a reboot of the Midax Fuel software, as described here.