I had a tiny leak from the right side of this fitting (see pic) that I barely noticed since it would accumulate into droplets and fall once in a while.

I've soldered copper once before and figured I could do this, but I ran into a problem.

I shut off the water and I think I drained this part of the house: opened a faucet both above and below this pipe until no water was coming out.

I'm using a TS8000T torch with propane and holding approximately the middle of the flame to the junction between the coupler and the pipe, right where the old solder is. I'm not using anywhere near the maximum flame (maybe 40% of maximum).

Seemingly no matter how long I heat it, the solder doesn't melt and I can't pull the pipes apart. I googled the melting point of common solders and they seem to be around 750-850F. I used my infrared thermometer to measure the temperature while heating and I couldn't get the coupler much above 320F. It is part of a large pipe system, so the heat definitely spreads out in both directions to some extent.

I'd like to give this another try, but what could be the cause? Does that 320 F limit suggest that there is still water in this pipe?