Well, as you know, make sure your metal is very clean, and it fits
together well, no gaps. Remember Solder will not fill gaps. The
two peices must be heated evenly. The entire peice must be up to
temp. Not just the small portion where the silder seam will be. And
of course that it is evenly covered with flux.
Sometimes it is just the luck of the draw wether solder will chose to
flow or not.
I personally wouldn’t even bother using plumbers solder, you should
be able to use Hard, Meduim, and Easy Solder, when doing marriage of
Good luck… Remember Practice Makes Perfect