I'm working in my old Philadelphia victorian rerouting the copper
supply lines, and I'm trying to tie into some older copper line with
new stuff. But the joints keep failing on the old copper side. The
lines are bone dry, and I've sanded them down to shiny copper so the
fittings slide on nicely, I've got it all fluxed up, but the solder
isn't taking. It looks like it's even being repelled! Any ideas why
I'm having such difficulty? All the other joints, where the copper's
all new are just fine.

Anrew