GVA is right--you've got water in the piping, making it impossible to get the fitting hot enough to solder correctly. Is there a way to drain the water off above/below the fitting? If you have a leaky valve, then you might have to turn the water off at the main shutoff and wait for it to drain out. A Sharkbite is probably not a bad solution if you can't get it dry. And if it's domestic water, you can always try the old lump of bread trick. But in any event, you have to take the fitting off and clean/flux everything down again before you try to resolder. If any of your solder stuck, you might need to cut it out. That's usually the easiest approach. Starting over from scratch--happens to me all the time. Kind of like restacking wood when the pile falls over. That happened to me yesterday. As to the reason for the failure--who knows? Pipes crack and leak over time. They seem to be making the walls thinner these days, so a little abuse with a pipe wrench is all it takes. I don't know if it's the right way to do it, but I always try to put the wrench on the fittings, not the pipe. And I managed to crack a fitting or two on my last project. Be especially careful with brass valves.