PORTER CABLE NAIL GUN (by 2 users)

I have a Porter Cable Bammer 6020 finish nailer. I ran out of gas and purchased some fuel cells on the net. I loaded the gun and it fires once. I have to lock the gun and pump and squeeze the trigger three times, unlock the gun and it fires once. I then have to repeat the process. I have tried all three fuel cells I purchased with the same result. Is the problem in the gun or the fuel cell? This Gun has always worked perfect for me. I can see nothing outside that looks worn.
Thanks
hornba@fuse.net