Post a picture of the lower receiver. Top view, side view, internals as mush as possible, and where the buffer retainer/tube meet.
Edit:
Are you using factory ammo or reloads? Can you cycle through a few rounds by just pulling back the charging handle? Did you have the headspacing checked?
Edit:
Are you using factory ammo or reloads? Can you cycle through a few rounds by just pulling back the charging handle? Did you have the headspacing checked?