Originally Posted by liverwort
If a good cleaning doesn't help, it might be that the recoil from the left barrel is pushing the shell back against the breech face. Try a putting piece of scotch tape under the rim and see if it fires. If it does the firing pin may be too short or the shells you are using have a rim that is too thin for the gun. or primers to deep, or someting else....

Yes. Maybe the rim cut is too deep.