The most common issue is that if there is ANY water in the pipes, they won't heat up enough. Once the water in the line starts to heat up, it basically goes toward the water in either water form or water vapor and then it makes it impossible to get the pipe hot enough or get a good joint. You also need flux on the pipe and fitting. It's definitely an art as much as a science. I upgraded all my water lines to 3/4" from 1/2" pipes about 3 years ago....I set my subfloor and insulation on fire a few times struggling with that torch in some spots.
They always say you could stuff bread in the water line to absorb the water/keep it from the fitting you're working on. In my system, my tankless water heater is the lowest point on the line. I just opened the drain on it and turned on a spigot upstairs after turning the water off.....then went to the end of the line with an air compressor and a threaded connector and flushed the lines with 60psi of air to blast the water out. Was a lot of work up front, but allowed me to work when I had to. I despise doing that kind of work.