Solder does not stick to tip of soldering iron?

First question: How long have you had the tip? You could probably easily and cheaply replace it. If the iron doesn't have replaceable tips, get rid of it!

If you've not had the tip long, then a few pointers to keep in mind:

• When the iron is not being used, keep some solder on the tip. This prevents oxidation and corrosion.
• When using the iron, keep a little solder on the tip when the iron is in the stand. Wipe clean before using. Avoid excessive wiping on a wet sponge as this can cause temperature fluctuations which expand and contract the metal, and stress the tip. Consider using brass shavings instead of a wet sponge.
• Never use sandpaper or abrasives to clean the tip. Also avoid dipping it into flux to clean. (Except for re-tinning which should be infrequent.)
• Use a good quality solder. Old solder can have contaminants which leave behind residue and basically make soldering more difficult. (By "old" I mean solder that's been around collecting dust, not necessarily that it has a short shelf life.)

Finally, if you've got a cheap iron, it may be too hot or cold and not melting solder at the correct temperature. Consider getting a temperature controlled unit. I could recommend a Hakko FX-888D for ~ $100 US, see also this tutorial.

Some additional soldering tip pointers:

• http://www.inlandcraft.com/uguides/tipcare.htm

My first guess would be that's just a low quality tip and no scrapping would help. There are some tips nowadays that arent properly coated. They're some alloy plated or even might be died with something and they're just for show. But in that cases whole iron is low quality also.

If it's some quality iron and tip (in its own time) and you scrap it too often it might be you overscrap it or it might even oxidation of inner copper alloy to degrade tip so it's for scrap. If it's HQ tip over years of use it should be replaced.

imho, there is no substitute for using tip tinner. You can find it easily on eBay or Amazon, and probably other places.

Just heat up the iron and push it into the stuff for a second, then wipe it off on a damp sponge.