When moving from desktop to mobile - size matters!
I have been knocking my head against this problem for a while (and just solved it) and thought that others might be interested...
In a little desktop standalone I made use of a tweaked version of Obello's excellent custom slider - which worked great. I was then asked to try doing a mobile version, which was fine except when I was testing the app on an iPhone 4 the 'thumb' graphic part of the slider would not slide unless I gave it an initial 'stab' to get it moving.
Eventually the answer dawned on me that perhaps the 'thumb' was too small for my fat fingers so I tried doubling it's size to 44x44 and it worked very nice without need for any initial 'stab'.