Just had to replace my keg coupler (dropped it and broke the plastic handles). I have installed the new coupler, a single lever piece. Beer is very slowly leaking where the tailpiece/hex nut connects with the keg coupler. I have washers in place, yet the beer (or foam) leaks out anyway. From what I can tell, it's not coming from the beer line itself. I do not have any plumber's tape on the coupler. Could that be the problem, and if not, what is?