Take a stick squeegee or a dowel rod and put it in the barrel resting on the bolt. Hit it with a hammer or some other blunt object and drive the bolt out. More then likely there is a piece of shell stuck in between the breech and the bolt, and because the tolerances are tight, the bolt gets stuck. The other thing that could be happening is that there is a burr or gouge in the metal of the bolt. You get it out the same way, but then if this is the case, then you have to sand the burr off the bolt (and maybe in the breech as well.)
But first get the bolt out and clean that really well.