Soldering soft copper to hard fitting
My guess is either the joint wasn't cleaned enough and/or not fluxed properly. I have had problems when my old tub of flux got dirt in it. When heating, the dirt burned and ruined the flux preventing a good solder joint. Check your flux and flux brush for dirt and replace or clean as needed. Then clean all of your joints thoroughly with one of those wire brush cleaners for copper tube or scotch bright.