I've taken apart some of my own test welds, and while most of them look
great (solder the length of the joint) every once in a while I do one that
I know is bad before it even cools, because the hot solder just keeps
rolling off the pipe instead of being sucked into the weld. When I take it
apart to confirm, I see that there is no solder in the weld, just a small
amount of black residue which I'm guessing is burnt flux.

Is that caused by something specific? Pipe too hot or too cold before
welding? Is it something that can be fixed if the weld is in place, or will
I have to always separate the joint and re-clean it, re-flux, and re-weld?
I'm thinking if the pipe is too hot, I should be able to let it cool and
then just try to scrape in some flux (it would get sucked in as the pipes
re-heat and it melts) and re-solder.

I'm thinking especially if this happens on a joint where I _can't_ tell it
is bad until it is too late (hard to reach valve installations, etc) that
it would be nice to be able to just fix it in place- I'm working on a
really old house and trying to put valves in-line in a couple of places I
can barely reach, and I'm not sure I'll be able to see well enough to be
certain that it is ok, and getting a valve back out to clean it will mean
taking apart 2 joints, doubling my chances of any errors when putting it
back (versus just fixing one weld).

Any advice welcome (other than "hire a pro", I wish I could afford ti but I
can't). Thanks!
Keith