Solder and flux compatibility?

[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

[imull]

Are you heating the ESC enough? Are you using leaded solder? Does the joint look shiney or dull? How hot is the iron? Are the wires tinned ok?

Soory for all of the questions, but they're the starters for 10