One other thing I was just thinking about now that you indicate yours is 2-3/4" too...
My 1100 is a pretty new firearm, just purchased new this past Spring. One change that Remington has recently made to the 1100 12 Ga. series (not sure about the 20 Ga., but can report that my sons new 1100 LT-20 youth uses a two piece piston) is using a gas piston that is a single piece unit. If I'm not mistaken, all older 1100 12 Ga.'s use a two piece piston assy. Don't know if this makes for more efficient operation or not, just a difference that I'm aware of.
And again, be REALLY SURE that the magazine tube (the tube the piston(s) slide on) is totally clean, especially where the little notch in the tube is at that holds the rubber O-ring in place. That little notch gets powder fowling in there that is the devil to clean totally. Pay special attention to just how smooth the piston(s) ride down that magazine tube. If this 1100 is pretty old and has fired thousands upon thousands of shots, you may be looking at a worn magazine tube and/or pistons as well which may require replacement to get it functioning perfect again. What's the black finish of the magazine tube look like? Is most of the paint/black color missing and you're down to the bright metal finish mostly? I'm not sure, but those magazine tubes may be impregnated with a semi-solid lubricant such as graphite that keeps them slippery (this is commonly done with all sorts of metal to metal contact situations). Once that black graphite coating is mostly gone, it becomes the dickens to keep the metal parts sliding across one-another efficiently, and efficiency is what you're looking for when shooting light loads - the smallest amount of resistance to the piston operation as possible. These parts aren't that expensive either if it boils down to that, and something that most anybody can do themselves without the aid of a gunsmith. Although a gunsmith would probably just have a moments look at it and tell you what likely needs to be done without any guessing.
Just throwing out some ideas...