not sure what went wrong,
but there's no way mapp-gas didn't have enough heat,
I've silver-brazed much larger pieces with mapp-gas,
sometimes I'll place a fire-brick behind the work to reflect the heat back in,
I'd throw out the black flux (its for brass-brazing, not for silver-brazing)
with white flux it turns transparent after it all melts and you can see
clearly whether the metal is clean or not and where the silver is flowing or not.
also paint the entire work in flux before starting so that parts you aren't yet
working on don't get oxide coated (which sometimes comes off with
additional flux, but if it does it usually takes an inordinate amount of flux),
also have a "scratch rod" (1/16 or so steel rod, pointed at the end and
last 1/2" bent 90-deg) on hand to push the silver around and get it to
wet to the parts if you're having trouble with that.
I always recommend against Oxy/Acet, its too hot for silver-brazing, you
run the risk of melting your pieces and/or burning the zinc out of the silver alloy.
and mapp is so much easier, just one knob to adjust the heat, with Oxy/Acet
you have to choose the right size tip for the amount of heat you want, and
play with the knobs to get a neural flame, you can't play with the knobs
for the amount of heat you want.
HTH, keep up the good work, love your Muncaster !!!
PS, not just writing for you, but also for other folks having issues with silver-brazing.