I have a cobra .38 derringer also. Bought new from a gun show. Fired it about 10 times over a year period. Took it out to the range one day and it wouldn't fire from the bottom chamber. It would dent the primer but wouldn't set the round off.
I called customer service (who was very helpful). I had some questions and asked to speak to a tech. The customer service woman (can't remember her name) put me through to a guy named Jake it think it was. He was very nice to talk to also. Gave him some info on the issue. He sent out some parts. Didn't fix the issue. Called back, same conversation as the first call, so he sent out some other parts and it was good as new.
I opted to have him send the parts to me because I had already taken the gun apart a few times and was comfortable replacing them and the closest fedex office was a 40 min drive.
Overall, the whole experience was very good. I feel that if I had sent my pistol in, the issue would have been taken care of quickly and sent back quickly just as well.