Marlin No. 29

[TWBryan]

Cleaning the guns in my father's collection I came across a pump action .22 with exposed hammer. Made by Marlin,model is No. 29. Took it apart and cleaned it up. It will load and eject cartridges properly. The issue it has is that the bolt will lock in place,either in closed or all the way open position. You have to jiggle the gun and use considerable strength to get it to close or open. I'm wondering if anyone knows which part of this rifle is not working correctly so I can start hunting a replacement part?

Thanks!