What is your hand position on the handle bars? Do you have any history of wrist or neck issues?
I've struggled with wrist tendonitis for years now, and I can't ride with my hands flat on the top of the bars. (Originally used it as an excuse not to do spinning ~ that didn't last long, and now we're on the road!) I have to ride holding the outside of the bar or in the hoods, so my thumbs are facing up & there is no break in my wrist - just a flat line knuckles to elbow.
Might also have something to do w/ bike fit, reach, etc...