Ok then, something to try, cycle the marker and hold the trigger so the bolt is back, now drop a ball in and release the trigger. Now drop another ball in the feed tube now fully cycle it.
Does the ball in the breech fire and the ball waiting to feed drop in normally . .. . or does the ball waiting to feed jump out of the top of the feed tube?
If it jumps out (and it depends on how high it jumps out) you've got blow back problems going on which is most like a timing issue. If the ball in the feed jumps bout 6ft into the air then you've most likely got a knackered 'o' ring on the bolt causing huge blow back.
But it does sound as though the sear isn't releasing the hammer on every cycle, so it's cocking and balls are feeding it but the 3 way or hammer lug has slipped so it isn't firing every cycle. A cocker cycle should be, release hammer (fire), open bolt, close bolt, release hammer etc. etc. make sure this is happening just pull the trigger slowly to check.
Hope that hopes!
Steve