2001 Ford F350 Blown spark plug.

I have a 2001 ford f350 and one day after coming out of the gas station I heard a loud pop after starting my truck. Upon inspection I found that I had blown out one of the plugs. The gap was completely closed and it had worked its way out untill it just popped the rest of the way. After I waited to get the tools and parts from people there I put the new plug in with no problem at all, it went in just as any normal plug would have. I then drove over 500 miles back home with it and it wasnt untill I went the change the other plugs that my "Problem" has occured. I went back to the hole where the plug had popped out to check and make sure there wasnt anything wrong with the cylinder, and there wasnt, its fine but now I cant get the plug back in at all. I have tried everything from new plugs to the old plugs. Ive gotten a 14mm chaser down in the threads just fine no problem at all. Ive also gotten a 14mm 1.25 tap in there just fine as well. I know that the threads are okay enough to get the chaser and the tap in there so why cant I get a plug down there? Any ideas? I appreciate any help I get as I have been at a loss of them since starting this.