One of the things that I have found is that flux pastes are designed for different uses. You may have a flux paste that is not the best thing for your application. Is it an actual paste or a sort of jell? For me the jell flux works better for wire soldering. There are also different types of flux core solder. You have to use solder designed for wiring applications, not plumbers solder. Plus you have to get the both items that you are soldering hot enough to let the solder adhear to the metals. Try to get someone that you know who knows how to do this to show you the tricks to the trade
. It is easy once you get the hang of it, but there are little quirks to getting it done right
. Reading it from instructions is not the best way to learn
. Seeing someone else do it or watching a video helps greatly
. Good luck with that
.