Take this for what's it worth as I have built 3. It may not have enough pressure to bring the bolt back. Some things to check include a misaligned gas block, a gas hole in the barrel that is plugged or too small, perhaps the buffer spring is too stiff or the trigger springs are too stiff.
Check out the video on Youtube "How An AR-15 Rifle Works: Part 2". Sounds like the disconnect is not catching when the bolt cycles so the hammer is following the bolt forward.
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