BREN L4A3 trigger will not move it full auto but work fine in Semi Auto

I have built an BREN L4A3. When the lower receiver is removed from the upper receiver, the trigger works smoothly in both semi auto and full auto. With everything assembled, I can easily pull the trigger and release the bolt in semiauto. If I switch the selector to full auto, the trigger locks up and can't be pulled. Sometimes, if I squeeze the trigger again (or a couple of times) the bolt will release. Any ideas as to what might cause this?

Not a good description of the fault, but...... Clearly, if the bolt is released on SEMI, but NOT auto, then the selector lever is not depressing the tripping lever sufficiently to clear the piston post when it's set to AUTO