Is this a takedown model? If so, check the juntion of the magazie tube with the action. If thre is a misalignmnt there the shell can hang up on the exposed edge of the action until you put enough force on it to cause it to ride up over that edge. Just opnen th action, with the magazine unloaded o course, and push the follower forward little and fel with your finger for any mislaignment. Also, If you have a dumy round, it's pretty easy to see into the action while working th pump to see any hang up.