Pahoo about covered it all.
Common causes of R-1100 failures are:
- Improperly replacing the rings (piston and piston seal). There eight different ways to put them on the mag tube and only one works. Verify ring orientation with your owners manual.
- Dirty/corroded/unlubricated mag tube. The mag tube should be meticulously cleaned and lightly oiled. As you turn an assembled 1100 end over end you should hear the rings freely sliding on the tube.
- Dirt in the barrel vent(s). This is a part of 1100 maintenance that's easily overlooked. As Pahoo noted, make sure the vent ports are clean. Use some sort of punch to get any carbon build-up out of the vents.
- A damaged barrel seal (O-ring) will shut-down your 1100 in an instant; but, it's usually the first thing most folks look at when trying to make a naughty 1100 behave. If you have any doubts about your O-ring, replace it.