I haven't had that problem with diet ice cream. Do you always put all the ice cream in the same part of the freezer? Maybe the air isn't circulating well in that area.
I'm having a problem with my five-year old refridgerator freezing things (mostly stuff placed near the back of the fridge, but also eggs). The manual said to try cleaning out the coils in the back of the fridge.
My Before and After Pics
"Decide what you want; decide what you're willing to exchange for it; establish your priorities, and go to work." --H.L. Hunt
"Life has hills. Set the treadmill at an incline."