I answered this over on that "other" wire, but ........ It is a common problem. It shouldn't be, but is. There is suppose to be a nice tasty bevel on the back wall of the Carrier Block mortice. Sometimes it's there. Sometimes it's too shallow. Sometimes it isn't. It will need to be added.
There is a nice tutorial on the procedure on Pioneer Gunworks web site. Give it a look. You'll need a nice sharp (new) small file. the bevel serves to prevent the cartridge rim from becoming trapped and it moves the cartridge back into the carrier block.
Coffinmaker