Plumbing solder is solid. It does not have an acid or resin core. Flux core solder can work on open joints such as twisted wires or a copper seam, but with plumbing work the flux has to penetrate into the joint and the core solder flux will boil away before that happens. That is why you have to preflux the tubing and fittings before you solder the joint. Any good plumbing manual will tell you not to use a core solder for sweating pipes. If you are having a problem with the “plumbing” solder, it could be 95/5 solder which requires more heat and solidifies faster than 50/50 or its equivalent lead free solder.