A tip I learned from my Dad, who was a plumber for 50+ years.
When soldering copper pipe, if there is any water dripping in the pipe to the place you are soldering the solder will not melt and flow as the spot will only ever get to 100 degrees C so long as there is any amount of liquid water. Try shoving a small amount of plain white bread into the pipe a couple of inches before you put the fitting, elbow, coupling, "T" or whatever on to be soldered. The bread will soak up the water drops and allow the joint to heat properly . When you turn the water back on, the water will dissolve the bread. Works great ! Nothing like creative thinking and experience in any trade.