you may want to check that the upper is recoiling far enough to clear the plunger on the sear. If it does not go far enough back than the plunger will be pushed back by the trigger bar and not reset. I've seen this happen on mismatched guns. I recently swapped the uppers on my 06 Bern and an 06/29 and found this to be the case on the 06 Bern with the 06/29 upper. It won't reset without a bit of fiddling...The plunger may be broken or have slipped forward some so that it won't allow reset too.
Check that and all the other stuff suggested and you should have a fix....
Biff
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