You are not supposed to be stopping it from sticking to the iron. Tinning the iron is an important part of getting the maximum heat transfer between the iron and the work. The soldering process means that you bring the work up to the melting point of the solder as quickly as possible. Then you melt the solder onto the work (using flux to force the solder to stick to the part) and then remove the iron to allow the part to cool down.

Clean the iron using gypsum or wire wool (don't file, especially with plated tips) or tip tinner, and immediately apply solder to the tip. If you have done it properly, the solder will adhere to the tip. If you have done it incorrectly, the solder will melt in blobs and fall off.

As you solder, apply solder to the tip, apply flux to the part, then put the solder iron on the part.

The solder is supposed to stick to the iron. You are soldering incorrectly, I suspect.

When you first heat up the iron, wipe it with a rag, then apply solder until it has an even coat. Wipe away any excess.
First, to solder a connection, get a strong mechanical connection by twisting the wires or other means.

Apply the iron to the connection while feeding in the solder until the solder flows around the joint. Move away from the iron. Hold the connection still until the solder solidifies.

Wipe the excess solder and contamination into a damp sponge. If the tip is corroded, try to remove the contamination with a wire bruise, then re-tin the tip. If that fails to coat the tip, replace the tip evenly. Your problem seems like a corroded tip, based on your limited description. Good luck.

Apropos "(using flux to clean the work)", the resin flux used in soldering doesn't, and isn't supposed to, clean. It is used as a barrier to prevent air from coming in contact with the joint and oxidizing anything during soldering.