Flux gang reporting in!
Flux and temperature are the secret to effective soldering.
I once had the dubious honor of fixing an air band radio fishing up in Alaska with only flux made from pine tree sap and a hot nail(dropped it on rocks snapped off antenna, had to make a wire replacement and solder it to the board).
Flux makes soldering work and if you cant get solder to 'stick' it is probably an oxidized surface preventing the solder form flowing properly.
Once it is all done you can clean up the flux mess with high % alcohol.
Liquid flux or flux pen for hot air/oven flow in soldering though solder powder with flux paste is used to get surface mount things to 'self solder' down from the heat while the little boxes and tins are for through hold soldering with an iron.