I have heard complaints from many people that they couldn’t get the solder melted immediately on the spot where they want to solder. That’s because either the tip is not clean because of oxidant and burnt flux from the previous soldering or it is clean like the picture below after cleaning on a wet sponge but in this case, the contact surface of the tip and solder is not large enough to initiate melting the solder and while you are struggling to melt it, it starts form an oxidized layer (not shinny).
So, right after you clean the tip on a wet sponge, touch the tip immediately to the solder gently and this will make a small blob of solder as shown in the picture below. Now there is melted solder on the tip, it is easy to start soldering. Give it a try! You will see what I mean.