This is what we call incomplete wetting of the solder joint: the solder didn't wet the surface well enough. This happens because solder has a surface tension of its own, the solder surface itself isn't always "sticky", and because the bare metal you're soldering to might have a thin layer of oxide or dirt on top that repels the liquid. On top of that, the solder needs to be at the right temperature – if it's too cold, it will not form bonds with metal surfaces.
This is usually solved with cleaning and using flux – flux is a component that reduces the liquid solder's surface tension. Your solder has a flux core (there's solder without, but you'll rarely see it in retail), and that should often suffice. If it doesn't here, it might be worth using a desoldering wick or pump to remove the old solder, clean the contact with a paper towel and a drop of alcohol (Wodka works... but isopropyl alcohol is worlds better and contains no potentially corrosive water) and use fresh solder. Don't overheat it – overheating it not only oxidizes metal surfaces and damages the PCB itself, it also burns the flux, which is the opposite of what you want.
In your specific case,it looks like the solder stuck to a few parts of the contact, that's good! so, remove the solder (heating it with a freshly tinned soldering iron that you removed excess solder from to "wick out" the solder on this joint might be enough), clean the contact, put your soldering iron to the contact and with the other hand feed in some fresh solder. Should do the trick.
Also, what greatly helps is liquid flux, which you can buy in little flasks, that comes with a tiny brush. Really makes wetting contacts a lot easier.