All you need to do is hone/polish the chamber and you're back in business. There are various procedures that are probably more effective, but I doone that isvery simple.
Remove the barrel and clean the chamber area. Now you need a cleaning rod that comes apart (like one of the cheap rods at wal mart). Just use one of the pieces of rod and put a brass cleaningbrush on it, smaller than the chamber (ie 20ga brushfor a 12 ga chamber, 410 brush for a 20ga barrel). Put this rod in a drill. Now you take some fine steel wool and wrap it onto the brush - enough to make a firm fit in the chamber. Spray plenty of rem oil on thesteel wooland in the chamber. Now just place the rod in the chamber and feed it in and out a little wayswith the drill at high speed. Do several 30 second increments, adding more rem oil in between. After a few minutes of this, clean the chamber again. Put very fine steel wool on this time. Repeat the procedure. Clean and oil the entire bore as you would usually. Reassemble the gun, and you're done.
Tinylines and ringsin the chamber of the shotgun will grab onto the shotshell's case when it's fired and expands. Polishing the chamber's surface to make it mirror-shiny will reduce this effect and, in effect, cause ejection to come more easily.