Over the years since the gun was built the wood may have lost moisture & shrunk. This could result in the sideplates being pulled in too far when the screw is tightened putting the works in a bind. Should this be the case the above suggestion of loosening the plate screw/s should also relieve the situation. If that works then you have but to determine whether its binding from the wood fit or the plate being sprung & then fix whichever it proves to be.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra