Hey blinko,
I have had the same problem with my genesis. I had a roto start but the thing fried so a reverted back to the pullstart. Somtimes I would get no resistance in the cord whatsoever and sometimes it would catch. So I took the back plate off the motor where the one way shaft meets the crank shaft on the piston. There is a spring and a pin that fits in the shaft of the piston that engauges when you pull the cord. My spring had fallen into 3 pieces allowing it to catch some of the time, but it was a pain. I called cen and the warranty covered it. It is very easy to install, look at the manual for the part I am talking about (G70357-11). I think useing the rotostart caused this part to go bad.

Good luck
AZ_JAY