Beginner Question - Solder won't stick to a certain wire
Try first sanding with fine sandpaper, or carefully scraping with knife.

I've encountered high-temp WW wire which simply cannot be tinned.  It does work OK if sanded first. Very abnormal. Something in the plastic insulation either coats the metal, or produces a thick (and invisible) tarnish layer.

IF NOT SANDED FIRST, then with lots of time, heat and flux, it eventually will tin.  (Time, like 20 or 30 sec.)  I don't know if this indicates thick oxide being reduced, or thin polymer being burned off.  What a pain.  And it wasn't some fly-by-night source; one of the normal suppliers IIRC Alpha wire.