At the risk of stating the obvious.......

"washer first, then the O-ring, tombstone holds it all together!"

Just check it again, it's possible you've assembled things in the wrong order or lost something.

__________________
A5 E-Trigger, JCS Red Hot tube, Full Vortex Mods, Squishies, JJ and Tru-Flite barrels, home brewed shrouds and Tac-cap, Car Stock, RAP4 raised sight mount, B+C red dot, polished guts. Push button WAS. Airhog Grips. Remote. HPA.