You're stuck with it, I'm afraid. Clean it well, use lots of flux, and get that copper hotter than hell is all the advice I have.

Or, you can do what I've done before, which is to cut out the whole offending section of pipe on either side of the trouble spot, sweat the problem joint while the pipe is loose and there's room to maneuver it, and then reattach the sonofabitch with two new fittings. Depends on whether you can get to it or not, of course. And that's more work. But it beats a leaky water pipe you can't quite reach.

Thanks. What's-his-name--Billy Something?--the OxyClean guy whom I can't stand (and someone here on MeCha has a crush on, lol), is all over TV lately screaming about some new epoxy that can seal water in pipes... but of course I have serious doubts.

Billy Mays. He's just so darn excited!

I second BOP most excellent advise. Can't go wring.

or wrong, even

The problem may be that there is water in the pipe and, when you heat the pipe, it makes the water expand just enough to push the solder out of the joint before it hardens. Try fully opening any and all taps you can find that are connected to that pipe before you heat the pipe.

Nth'ing everyone else - Soldering pipes depends on capillary action between the pipe and the fitting - the gap should be small, cleaned well with crocus cloth, and well fluxed.

The idea is to use the heat to draw the molten solder into the joint. The puddle of solder will follow the hot spot a bit, so play the flame about an eighth of an inch / quarter cm ahead of where you're feeding in the solder.

Clean off old flux from prior attempts, too - it is the chemical action of the stuff as it burns off that works the magic; pre-cooked flux won't help.

It's BoringPostcards who has the crush on Billy Mays. I think he's cute too, but man, the guy needs to quit YELLING SO LOUDLY. We'll buy the damned product if you'll just shut up!

overthinking a bead of solder?

My problem with sweating a joint is that I always equate the brightest part of the flame with the hottest part, but you have to move the torch away to get to the hottest spot. I learned this when I redid my bathroom at Christmas time and finally called in a plumber so I wouldn't burn my house down.

FYI: This is a good Plumbing Forum.

The hottest part of the flame is right where it changes colour.