Mine are a little slow to snap back to position as well. I'd say it was a rarer, intermittent problem, but my issue now is with both shift buttons(on both levers). They occasionally lock up and are unshiftable? Campy says the reason is due to my hands resting on the levers while riding. Their advice is to push upwards on the shift button/lever until they become free. I've had no luck with this solution, and should resting my fingers on the levers completely lock up my shifters? As the shifters are less than a year old, I'm concerned about the performance in time...
What Campy is refering to is if you have even a small bit of finger pressure on the longer shift levers (behind the brake levers) at the same time you are trying to press down on the thumb levers then they will lock and won't shift. Sounds elementary but when you are knackered a bit you might not realize what all the digits (fingers) are doing at that point in time. In my case I know about it but still it happens maybe once or twice a season.....Other than this, my suggestion mirrors what others have said - check that the hoods don't create too much of a bind and that handlebar tape (wrapped around the clamp on the back side of the lever) isn't also causing the problem.
It was explained differently to me by Campy, however, what you are saying makes more sense. The Campy tech said he had the same problem of resting his fingers against the shifters while riding, and had to push outwards or upwards to free them when they locked. In my case, my shifters periodically lock when pushing one lever or the other, not simultaneously as you explained. For them to unlock I very deliberately have to push each lever/button independently until it unlocks, not sure exactly how it happens?
The hood was rubbing a little on the shift button, appreciate the tip there. Sorry to the OP for hijacking the thread a little, but I tried to tie it back in to the original topic. Hope it works for out you.