lawnboy 10607 with sens a speed

I have a Lawnboy 10607 that has sens a speed. When I engage the cable to make to self propelled, it will not move at all. I have adjusted the cable, and it still just will not engage. Turning the mower on its side, I can see that the spring just does not move forward enough when pressing the handle, to engage. But I can bring the spring forward, with my finger, and it appears to engage (of course this is done when not running the mower). Anyone have any idea what could be wrong with the self propelled unit? Many thanks.