Over the years I have seen so many 1100s that their owners just can't seem to get to run right. I have a friend who is now far away, and he's a Beretta fan, and he has seen much the same thing. It seems it is always some small thing, and I have yet to see one I couldn't get running right except for one with a cracked receiver and drilled out ports (guy drilled 1/8" 11-87 sized ports in an 1100 barrel with no gas relief system - "I measured the ports in my other barrel!?").
If there are two shells in the magazine and the second one doesn't feed, it could be the trigger group and/or the interceptor latch. If the gun is loaded with at least three shells in the magazine, and cycles all but the last one, that rules out trigger group, shell latch, and/or interceptor issues. Which means something is amiss in the magazine. Which narrows it down to crud, a messed up follower, a weak spring, or a combination. If the follower is plastic and the receiver end of the inside of the tube is rusty, it can drag. Unless a spring has gotten rusty and been cleaned off and is therefore weakened, I haven seen a spring by itself be too weak to cycle the action. I have five 1100s at present, have had 12, and have yet to replace ANY springs in any of them.
In this case, I would first remove the plug and load three shells in the magazine and check that out. If the first two cycle, I would then smooth up the follower with fine sandpaper, and wrap a bore brush with steel wool and polish the inside of the mag tube with an electric drill. Do NOT got too far because if that snags the shell latch you will have a train wreck. Good luck.
An aluminum follower isn't a bad idea if you need to replace the follower. The original steel ones are good too, but they will eventually batter the receiver. On my '63 the nose of the follower will occasionally just catch the tip of the carrier after the last shot. As long as you clean the gun properly you shouldn't ever have a problem with the mag tube or the plastic follower either, but apparently that doesn't always happen.