I had the same issue hen I first got mine and yes I too sorted it by changing the thumb size. I have set mine so that to pitch bend down I just roll my thumb down from where I naturally have it at the thumb rest of the instrument. Then if I want to pitch bend a note upward I start the note with my thumb in the roll down position and roll it back up to the thumb rest of the instrument. This seems to work well although sometimes the slider seems to loose its start setting and the pitch bend only works halfway down the slider. When this happens I lift all fingers off the sylphyo for a few seconds at the first opportunity, then carry on playing. This seems to recalibrate the setting and all is good again.