Years ago I could not get a copper pipe joint to take solder. 5 tries and it still leaked, Aarrrgh! Turned out the liquid flux was old. New paste flux and presto!
But that was plumbing. Today I had to solder the electrical connections on LED strip lights. I had never done much electrical but believed that the old rosin core solder would work. After much frustration I decided to put on a dab of paste flux and presto!