1) Has this happened recently.
2) Did this happen after reassembly.
I would pump it and do a disassemble to see where the bolt lines up. If the bolt is completely flush then it has to do with sear or trigger. If the sear looks normal, then inspect the trigger. If the bolt isn't flush forward something it interfering with pump mechanism.
~Carter Buzzard 96'. Freak Custom Kit. Still not done....
~Angel Ir3. Freak. Lo-rise clamp. A4 bolt. Ti hammer 45/45. REV x-board & ghetto intellifeed .
SLATE Phantom. HPA. ANS reg. clear parts. Lighten hammer