Are the places where it isn't flowinmg oxidized or stained somehow? Try a little flux. It helps the "flow" go sooner and with less heat. Also if you melt solder on your tip and expect to have it the same qualities after more than ten seconds....it doesn't. Melted solder will oxidize after ten seconds and the "flow" won't happen without severe angle changes and pressure. Wet the tip on a moist sponge to clean any foreign matter before every solder joint attempt, wet the tip with solder, put the tip opn the joint to be soldered, apply a tiny bit more solder to start the heat transfer, wait a bit for the heat to transfer, then apply some more solder. It should flow to both objectives and leave a nice convex radius not a concave as you desctribe.