When soldering on copper, it is also imperative to have a heavy-duty
flux, and to make every effort to get the piece up to soldering heat
fast. Copper oxidizes very readily, so you are running a race
between flowing the solder and saturating your flux with crud. I use
Stay-Silv paste flux, which is made for plumbers (and thus for
copper soldering) and it works well for me.
Dos Manos Jewelry