I had a problem like that and eventually removed all ptfe, cleaned the threads up and put a smear of Araldite on the threads,screwed it back in and left it to set. It hasn't leaked since.
I know that some manufacturers of stuff warn against this as it could cause the 'plastic' to deteriorate, so you do this at your own risk. The fitting I had the problem with was an NPT JG fitting into a BSP female brass fitting. In this instance I don't mind if the plastic breaks down as it is easily replaced.
I also didn't see any warning on the Araldite instructions about gluing restrictions, so just presume that some manufacturer is covering themselves for every eventuality.