Operate it from under the hood with a stick and see what is happening. I started my 235 many times this way while testing things/timing but I had the clip off. Your foot pushes the starter gear into mesh with the flywheel and also makes electrical contact. The mechanical portion (linkage and sliding gear) should be easy to detect why it's not driving flywheel. If you see it always slide into the flywheel gear and sometimes still does not spin flywheel then the little gear is loose on shaft or stripped. A partner would be easier....and a good flashlight! YOU HAVE TO watch what is happening at flywheel/starter mesh.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn