Soldering help - RPi 0
Possible reasons for not being able to melt solder:


Small footprint for the pad makes it harder to transfer heat.
Crappy solder iron.
Too small solder tip.
Pad is efficient in dissipating heat. It could be that it is part of a bigger copper pool which can consume the heat applied and and radiate it elsewhere. Perhaps it is connected to GND?
Oxide on the solder tip will make it hard to transfer heat.
When manufacturing a board in a fab, you cannot and should not use any lead based solder. Non-lead solder does have a much higher melting point and makes it difficult to melt.


Advice that could help you:


Get an smd hot air gun or similar and pre-heat the board before trying to solder.
Apply flux.
Get a good soldering iron and a pretty large tip and set it to about 400C.
Make sure that the tip is free from oxide by using a metal wire steel ball.
Before soldering put some 60/40 on the tip itself so that you get a better connection between the two surfaces. Also if you are lucky you will get the lead into the joint itself and act as a catalyst to the melting process.