Recoil start problem
 

My Flymo Lawnchief 400 with the Tecumseh Vantage 35 motor has developed a problem with the recoil start. I've removed the assembly from the engine but can't see an obvious cause. I'm a bit puzzled as to what causes the dogs to engage with the engine. Inertia alone is not enough to cause them to pop out and there doesn't seem to be a friction device of any sort. I wonder if there should be and it has worn and disappeared. The machine has not had a great deal of use and is in general in very good condition. Any ideas?