I noticed the same thing while shooting SA in my FAL. I had switched the gas system off, and was ejecting the cases manually using the charging handle. With Australian surplus ammo, I could do this as easily as ejecting unfired rounds. With SA surplus, (which was very accurate, by the way), about 1 out of every 2 cases would "stick", and I would have to hit the butt of the rifle on the ground to free them.
Examining these cases, I noticed that the "neck sealant" had, upon firing, oozed slightly between the chamber wall and the case mouth, "gluing" them in. Apparently, there was a certain amount of force that it took to break that bond and free them, which I was unable to achieve by hand pulling on the charging handle. (Think of trying to swing a hammer when there is only enough space to move it one inch).
The SA ammo was designed for a FAL, where the tilting bolt is in a heavy carrier that has been accelerated by the gas piston to it's maximum speed before it grabs the bolt and starts to eject the case. In this environment it usually works fine.
I could see there being a problem using it in a rifle having a different style gas system, (M1A or AR-10), or one having a tighter chamber in the area around the case mouth. I could also see that different ammo lots might have more or less sealant, so you might want to try some with a different date.