I have had this happen to me on a shower facuet. When I soldered the
pipe, the heat effected the telfon tape and resulted in a minor leak. I
wrapped the joint that had the telfon tape (after replacing the tape)
with a wet rag so it wouldn't get hot when I soldered the adjacent
joint. A pro would be able o solder fast enough not to have this
problem but I am not a pro. Might be your problem,