bolt not locking back on last shot

[terrywick4]

I have an issue that I can't seem to get figured out. When I shoot my last shot my bolt doesn't lock back. I can pull the charging handle back on an empty mag and the bolt will lock back on it own, but when it is fired it doesn't? I Put a adjustable gas block on today thinking it might solve the problem. It did fix the dents in the brass but It still doesn't lock back. I tried the upper on a different stock lower (colt law enforcment)and still doesn't lock back. Any Ideas?