I believe that there is a "slip clutch" in balt advancement system, similar to what's inside a Maverick, Strongarm, and just about every other turreted blaster. This helps prevent things from breaking when the belt gets stuck, but it also doesn't let the vulcan pull the belt very hard.
If your belt is hanging straight down(i.e., you have no ammo box) then that is one problem. Hasbro gave it an ammo box so that the weight of the chain would be supported. Even with the ammo box, the blet can still get caught up as it's being fed. If you didn't get the plastic ammo box with yours, even making one out of cardboard could help.
Also I have heard of people putting a rounded piece on the side of the vulcan that the chain feeds into, in order to prevent the belt from getting caught up. I don't know of any pictures of this, but it might be worth looking into.