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.