Last week I had 3 freshly minted BSc engineers try to wire up a 128 LED matrix. They had trouble soldering because
1) They were using lead free solder
2) They were using an iron coated copper tip rather than a pure copper tip to which they were accustomed.
Both of these factors require more control in temp & tip/joint cleanliness.
They also ramped the 40watt iron to it's max temp in order to get the joint hot enough to solder.
By the time I got involved I had to use an exacto blade and scrape a heap of oxide and solder slag off the tip while tinning as I went along. Naturally adjusting the iron to the correct temp also slowed the rate of oxidation and permitted a better tip condition.
The moral is: Clean the tip, tin the tip, keep a moist tip cleaning sponge on hand and get the temp right!
BTW, if your solder wire diameter is too small it won't do a good job of tinning the tip. I have heavier gauge 1.6m to 3mm wire on hand to always tin when I am using the 0.8mm solder wire.