The selector will move to safe but only with considerable force...force that shouldn't have to be applied.  Works fine in all other modes but acts up when bolt is locked back.

Any suggestions?

Nobby

Sort of a stupid response, but why are you moving your selector when your bolt is open?

I don't have my AR in front of me but here's something you can try. Separate your upper and your lower. On your lower, push down on the hammer and try to move the selector. This will simulate your BCG passing over your hammer and cocking it. You should be able to feel when the resistance builds up. Try to see what's rubbing against what and if it's a problem.

Here's a dumb answer: I instinctively move the selector to "safe" after the last round is fired.   It's a safety precuation that ensures the rifle is safe when the first round of a fresh mag is chambered.  When I'm ready to fire, I simply move the selector to "fire" and presto.  

I will try your suggestion.  Thanks.