the cranking pressure is down in that cylinder. Could be a broken or damaged ring. Or there could be worse things.
I knew quite a while ago that there was too much pressure in the crankcase and it was probably being cause by blow by. That would be a good reason why the PCV blew out of the engine when I last ran it at Epping during the Street Stang race. That was almost 2 years ago. Remember that one, you were there.
That kind of stuff could also happen to a NA engine. You start beating on anything and eventually something will give.