Here some thoughts:

First....could the coil spring inside the pull-start mechanism be slipping? The ends can slip for a variety of reasons dependant on the design.

Second......every slipping OWB I had seemed to work perfect when I tried to turn it with my fingers. In fact, it usually wouldn't slip if I used the pull-start slowly...but so slowly, the engine wouldn't start.

Third...despite the fact you changed the OWB, my only experience with OS OWBings wasn't good. Brand new OS 30VG and the OWB lasted one bash session. I switched to a HPI "Made in Germany" OWB and it's been fine since.