Out of the USMC manual:
1) weak or broken action spring
2) unlubricated or dirty action spring and receiver extension(buffer tube)
3) improper gap space or worn, missing, or broken bolt rings
******This is what I would check first. Look at your bolt rings----are they lined up together. THE GAPS SHOULD NOT ALIGN, at least not all three. This will cause short stroking and is very eay to fix. Also, did you put the rings on the bolt?. If so you might have bent one. Just a thought.
4) gas leakage caused by broken or loose gas tube around front sight base.
*****This I'd check third
5) improper alignment of gas tube and carrier key
Also consider that if you built it yourself the trigger group may not have been assembled in the correct fashion(ie correct order with springs in proper places) and this might result in hammer not striking with enough force. Hammer springs should sit on TOP of the trigger pin, for instance.What did you use as a reference to build it?
***********************This is what I'd check second. I've got a feeling this is it, though, given that you have to push the trigger forward to get the trigger/hammer to function.
Are you sure you put the disconnector spring into the trigger before placing disconnector in?
Does the hammer go forward? are you sure? Hammer catching on the disconnector is normal provided it lets go at end of trigger second stage.
What kind of mags are you using?