If the solder just rolls off the wires, then they arent getting hot enough. Yes, they need to be really hot. Sounds like you might need a hotter iron. The iron will melt the solder WAY quicker than the wires will heat up enough for the solder to stick.

My technique is to heat the hell out of the wires first then touch the solder only to the wires, not the iron. If the solder melts by only touching the wires, then it most likley will stick. But that sounds like what yer doing.

Contamination may be an issue, but usually the solder itself has enuff flux to do the job. When in doubt, cut back more wire, strip again, and rough up the surface by scraping the bare ends with a knife, it may help. I've also had some pretty crappy solder at times too, so that may be at play here.