My 1100 magazine became stuck a couple times, and I couldn't put a shell in, but it just needed cleaning and lubricated. The shell wouldn't start, though- because the magazine follower was jammed.
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This might sound silly but is your shell swelled up. Look to see if anything else may be wrong with the shell. i got some 20ga shells that had been crushed so lightly and they wouldn't fit in the mag or the chamber.