My Stihl hedge trimmer blades bound up and wouldn't stroke. I couldn't find any debris causing the problem, so I tore into the gearbox. I ended up finding worn lobes on the spur gear. The worn lobes reduced the mechanical advantage (or leverage) the gear was able to apply on the blades during rotation. I also found one of the blades broken at the driving end. I repaired the break and built up the edge with a welder. A grinder and file got me back to size. I hope you'll find this repair useful if you come across something similar.

Amazon Affiliate Links
Husqvarna Hedge Trimmer https://amzn.to/46BD1Br