If the shell simply won't move after firing the first thing I look to is the chamber itself. Many of those shotguns have chambers so rough that it causes the shell heads to cling to it upon firing and it makes extraction very difficult..
A polishing session, getting it mirror-bright with a Sunnin hone and various wet/dry papers will usually cure that issue..
Most of those Win pumps actually require a little forward pressure before you eject. .
That applies to the M12.. But the 120 (and 1200) use a different system to unlock the action arms; basically as soon as the hammer goes fully forward the flat section of the hammer pin causes the disconnector to drop and unlocks the bolt/fore-end..
Since it works ok with unfired shells I doubt the problem lies with that area.. More likely (IME) it's a chamber issue..
