It looks like you're overheating the joints and burning out all of the flux. The magic to this is move the flame, test periodically to see if the solder can melt yet, and when it does, remove the heat, apply the rest of the solder then leave it alone to cool. If you're handy and can do it fast enough, you can use a rag to wipe excess off for asthetics. Wait too long, though and you can create more problems by maybe moving things as the solder is setting.
Personally, if you don't do this often, I've found differences in the fluxes available - some are much harder to get a good joint with than others. One often mentioned as being more friendly is a tinning flux - it has not only flux but powdered solder in it. You still need to add solder to get a waterproof joint, but it sort of pre-tins it for you to help things along.