Gil, from my own experience the tails of the sears have to be absolutely level with each other, and the trigger mechanism must be in exactly the right position in the up/down plane..
This can be very frustrating with a M21 because the trigger mechanism is so precisely made. I fixed one of mine through careful study with a magnifier. I made up a dummy bushing of the exact length to simulate the stock bearing, and studied the trigger action while the trigger plate was installed but the stock was off the gun. All it needed was a stoning of one of the sear tails to get it to align exactly with the other one. I doubt that I took off more than a thou or two. Possibly that sear had worn very slightly through use causing the misalignment. CAVEAT: if you stone too much off either sear you're going to have more problems.
You might try tightening or loosening the top tang screw to see if that helps. Same with the screw that's in the rear of the tang under the trigger guard.
Possibly the best advice I can give you is to send the gun to Galazan/Connecticut for adjustment. Silvers
