I too would think that your finger is hitting the button if it is hard to push the selector button over to the other side. Your finger will ride on the button if you use the first joint/knuckle of your finger to pull the trigger. So you might also want to make sure that you use the pad of your finger on the trigger rather having it down to the first joint/crease of your finger. Have you tried to see what happens when you shoot the left barrel first and shoot the gun right handed? My buttons on all my SST 21s are really hard to move. Maybe the impact of the recoil plus a slight riding on the button are doing it. That would be my guess, too.
