LOTS of possibilities If the above hints don't work out. Someone might have been bent on "improving" the action. Don't be afraid of sending it n to the factory on general principles if the solution isn't readily apparent.

I have a personal point on DA Smiths beyond which I will not go. Then I box it up and send it to the real experts. I've done that with four guns in the last few years and been impressed with their diagnostic skills, quality of work, and the reasonable cost.

Not to mention that it's much less stressful.