Soldering station iron tip won't tin.

[Monkeysmate]

Ok. I've got a decent soldering station, and one of the tips won't tin. It will get up to temperature, but the solder just beads off it. What can I try? I've tried flooding it with solder, and cleaning with a couple of different fluxes, and giving it a rub with wire wool, but no joy. All the other tips work fine.
Any suggestions gratefully received.
Thank you