"I don't know why I know this but I do" thats scary dude...lol
Serioulsy if the fitting doesnt seem like it wants to tighten up , put alittle extra teflon tape on it or just toss it and try a new fitting, sometimes they get alittle carried away tapping them fittings and run it to far down the fitting which will keep it from being able to tighten up all the way. I prefer the shouldered brass set plugs over those allen ones and also I use a combination of telfon tape and pipe dope.
Goodluck, Charles