Either there is still some water in the pipe - or more likely it's
Silver Solder or Lead Free Solder, both of which are nearly impossible
to melt with Propane. Buy or rent an acetylene torch, which are far
hotter, and the pipe will come apart very quickly.

Interesting. I've never had a problem sweating copper using a propane torch
and lead free solder, the soft stuff anyway. Another alternative to propane
may be MAPP gas? The plumbers who worked in our house used that, must be
some reason, probably heats faster, thus saving time?.
Looks like it's a bit more expensive than propane, but less costly thatn
acetylene.

Dave