Looking at that picture, too little flux.
Certainly looks that way. Either the flux has burned/evaporated away, or there wasn't much before hand.
You don't need Louis' levels of flux for this, I just use a flux pen on the board.
I'm wondering if the OP keeps re-applying heat multiple times? Maybe because the iron's not hot enough?
Just a short dab for a second or so with the iron should be enough for the anchor pad.
Then a short dab with solder for the remaining pins. Don't apply solder to the tip, heat up the pad and pin for a second, and apply solder to the joint. Shouldn't take any more than a second or two total each joint. Clean your tip regularly in a wet sponge and/or that brass coily stuff. If there's any residual solder on the tip, the flux will have burned away, so get rid of it. Similarly, if you keep going back in on a joint, the flux will disappear. The joints should look smooth, not like an unsurfaced road ;-)
FWIW I use 350C to a 0.2mm conical, although I know there's a school of thought that prefers a chisel tip.