Egregoros

@sun Either not hot enough or you're touching the solder directly to the iron tip (probably out of frustration because shit iron + lead-free solder). Hold the iron on one side of the pad and be patient. Hold the solder on the other side touching the pad and touching the pin of the header you're trying to solder and it will flow once it's hot enough. Try not to touch the solder directly to the tip of the iron - that's how you end up with it balling up like that because the pad didn't get hot enough. If you're having trouble getting it hot enough to melt on the pad because your soldering iron just can't get hot enough then try using leaded solder instead of lead-free solder. Just don't breathe it in. Lead is good. Lead makes you happy.

Also pays to be a bit more patient when soldering the ground pin as a general rule. Usually they will use thermal relief pads to make it easier but sometimes they don't (especially if the they is me and I forgot )