In the restaurant I own we have constant problems with the Diet Coke on our fountain. I've had numerous techs from coke try to figure out the problem. Turns out that Diet Coke must be served at a temp of 35 degrees, whereas Coke, Pibb, and Sprite can be served at temps slightly higher than that. The temp of the Diet Coke affects its' taste and how well it holds carbonation. That's probably why a chilled CAN of Diet Coke (metal which holds cold better), tastes better than a chilled BOTTLE of Diet Coke. Of course the calibration of the machine and how often the dispensing nozzles are cleaned have a lot to do with it. Keep in mind that the nozzles where the soda is dispensed can become completely clogged, therefore changing the how well the syrups are mixed with the water.