I am a neophyte at soldering. I am building a manifold which consists of 3/4 copper tubing and cutoff valves. I am having trouble with one solder joint. All the rest seem to test out under pressure. It is where one of the cutoff valves connects to the 3/4 tubing. I have been heating the valve (with a torch) and trying to "flow" the solder into the joint, but it seems to keep "balling" up and will not get "sucked up" and does not make an airtight connection. I am using flux. What is the problem, and how do I solve it?