Been doing a bit of plumbing work lately and have come across a reoccuring problem three times. A few times I have soldered an incorrect fitting to a run of copper pipe. I try to remove the fitting by reheating, the solder melts and I pull on the fitting but the fitting jams and will not budge. Usually at this point the fitting looks pretty dry and free from solder/all solder on one side of joint. No matter how much I heat the fitting will not remove. My solution has been to hack off the fitting and couple on some new pipe. As an experiment I clamped one of these off-cuts in a vice and continued to heat the fitting to see if it would eventually remove the copper went red hot and as I pulled on the fitting it still would not move and the the pipe easily bent as it was so hot. I want to stop this happening. What would be causing it do I need to apply more solder or flux in order to remove these fittings?