grips not always heating up

[h2only]

my year old f800gs has recently had a strange issue with the heated grips. On either the high or low setting the grips will sometimes warm up but sometimes they dont heat up at all. I have noticed if I lightly rev the bike out in town, the grips dont want to heat up even after 30 minutes of stop and go traffic. But if I rev the motor into the higher rpms, then the grips work fine. Any ideas of what might be causing this?