If you are messing around with electronics and solder, do NOT use plumbing (acid) flux - EVER! You will end up with a nice looking solder joint that will eventually become worthless once the acid flux eats up the copper traces. Electronics solder has its own flux, which is usually a liquid or is embedded in the solder core.
Also, if you are going to add more solder be careful and do not add too much. "The bigger the blob, the better the job" doesn't apply here. If the joint is grainy and a big blob, remove the old solder first with a desoldering pump or with solder wick. Then try again with fresh solder.
Here's a nice link to a soldering howto:
http://www.mediacollege.com/misc/solder/