Your workpiece is not hot enough. Your soldering iron may not be properly tinned, workpieces dirty, wrong solder alloy, etc. 50/50 plumber's solder is inappropriate for electric/electronic work, because of the acid flux and the higher melting temperature. There must be a "heat bridge" of solder between the iron and the workpiece, when the workpiece is hot enough the solder will wet it, not "stick" to it.
Originally Posted by David Lyga