When you push on the thumbpiece (21) which is attached to the bolt (22) it moves the bolt forward againist the center pin (2) within the cylinder. The front of the center pin then pushes the locking bolt (66) forward, and at this point the cylinder assembly should be unlatched so that it can be swung out to the left.
If the extractor rod (1) which is really a tube, is slightly unscrewed the center pin cannot push the locking bolt forward far enough so the locking bolt is clear of the extractor rod's end, so you can't open the cylinder.
At this point I can tell you what is happening, or not as the case may be - but not why it is - without examining the revolver.
If the extractor rod is screwed tight I would suspect that there is something caught between the bolt and frame that's preventing it from being pushed forward far enough to push the center rod the distance it has to go to push the locking bolt clear of the end of the extractor rod. Annother possibility is a bent center pin, extractor rod, or both.
Yet another possibility is that the locking bolt spring (65) is so filled with fouling, dirt, (whatever) that it won't compress far enough so that the locking bolt can clear the end of the extractor rod..
If the cause and solution aren't obvious you may have to take it to a gunsmith, or better yet send it into S&W, where they have special pin punches and screwdrivers - as well as other tools - to completely disassemble and inspect the gun.