The iron tip needs to be "tinned" first, then kept clean but with a thin layer of solder still on it. Use the cleaner then immediately tin it and shake or wipe any excess solder off using a damp sponge (or damp cotton - bits of old denim jeans work well).
Note that Lead free solder is is nothing like as easy to work with as conventional tin-lead solder, it just does not flow as well.
The solder should also be a flux-cored type. I'd advise getting some 60/40 or 63/37 alloy, multicore or similar.
The thin layer of solder on the iron provides much of the surface area needed to heat the joint, so the solder you add to the joint flows quickly on contact with it. If that layer oxidises or burns away, the iron will not work well if at all.
Soldering works by forming something like a very thin layer of metal alloy between the solder and other metal; that can only work if heat from the metal is adequate to keep the solder liquid, and there is flux present to dissolve any surface oxides and allow the metals to combine within the joint.
A single sided PCB joint should flow almost instantly & a double sided still in less than a second, as there is more metal to heat.
(I normally have my iron at 350 - 360' for most things.)