[mrerick]

Assuming that the fitting of the take down lever is correct, there is a small "L" shaped spring that holds it in position inside the channel it fits in.

This spring may be worn, or have bent excessively. It has on side shaved along it's long axis causing it to curve slightly in order to fit into a groove cut in the side of the take down lever, which retains it for correct operation. It might be worth inspecting this for dirt or malformation.

This can be a very tight area normally, and it is possible to break the spring. If the spring is already broken, it could also cause this difficult operation.