Good advice above. Echoing it, I would:
--Check main mechanisms to make sure no parts are broke or binding. You can get some idea of this by pulling back bolt with lever to lock in open position, then releasing by hitting release underneath, then doing over again.
--Remove forearm and barrel. Play around with the piston/action bar assembly to see if it pushes back the bolt assembly without anything binding. Look for broken or bent parts.
--See if shell goes into and out of chamber of barrel easily (use a snap shell or empty shell). Make sure little hole or holes in barrel that vent gas into the piston are not blocked.
--Make sure the piston ring assembly is on the right way around. See if O-ring is intact and in its groove (usually will work even if not in the best of shape, at least on my 12ga). Check that piston assembly slides up and down smoothly on the magazine tube--i.e., not too gummed up.
Gas guns run dirty, and I've had gummed up works eventually stopping the gun from cycling, especially with light loads. Twice, I've had my 1100 12ga stop cycling because of broken parts--once when one "tine" of the link broke, once when a bar of the action bar assembly broke. BTW, when I replaced the link, I noticed the little cup the link "tines" go into to push back the action spring inside of the stock tube was chewed up (probably the aluminum version mentioned above); so I took off the stock and replaced that and the spring, too. The spring and tube actually were quite clean and probably didn't need to be bothered, but mine is an old trap gun not used for hunting.
Don't take everything apart unless really necessary. I suspect something quite simple and obvious (once you notice it) is broken or wrong. Good luck
