My grease tubes are always stored with the plastic cap up.  I also correctly load the tubes by starting a few threads and then releasing the plunger before screwing it in the rest of the way.  Yesterday, I was using my "main" gun with tubes of Blueguard.  I then switched guns to one that had an empty bluegard tube in it and put in a tube of polyurea.  It pumped half of it and then quit.  I fiddled with it for 10 minutes but pulling the plunger out and pushing it back in and removing the tube several times thinking it was empty.  Each time I tried it would work for 6-8 pumps and then quit.  Eventually I took the tube out of the "main" gun and everything worked great for 6 more tubes.....so back to the question......what goes wrong?  Is the plunger seal bad?  Is the spring bad?  I don't get grease past the seal that comes out the bottom of the tube and the spring seems to have adequate tension when I load a tube.