[Monkeysmate]

I'm having a bit of a nightmare trying to solder new wires to an old ESC...
I cleared of as much old solder of the old ESC as I could, but come what may, I can't get the new solder to stick to it...
Any advice please?
Is it likely to be a material compatibility issue?
Are all solders Mixable?
Are fluxes compatible with all solders?
Thank you.
Tim