A few drops of isopropanol (or, if you can’t get it easily, 91% Isopropyl alcohol from the pharmacy) may revive your flux. Don’t use the 70% “Rubbing Alcohol” from the grocery store “Beauty Supply” section.
I don’t recall the brand or formula for liquid flux they tell me to use at work. I only use it for tinning wires in the solder pot. @Sprig has his feces amalgamated but others stumbling across this thread need a reminder to use ONLY rosin-based fluxes or others intentionally formulated for electronics - NEVER the zinc-chloride stuff from the hardware store or hobby shop.
I don’t recall when I last used a standalone flux on a PCB assembly. For thru-hole parts it’s just a desktop, regulated, soldering iron (Hakko FX-888) and thin-gauge, flux-core, wire solder. If a large solder joint doesn’t want to flow smoothly I grab the soldering iron from the next worktable (and maybe a co-worker) and use two irons on the joint. For surface-mount parts I usually use the same solder paste that would be used in a reflow process. " CML Supply Paste Syringe " (and other vendors) make it available in small syringes. With solder paste under the part you only need to touch the smallest tip of your iron to the pad, then wait a second for the whole drop of paste to melt and flow. I know the paste advertises a shelf life of 6 months but the two-year old syringe I keep in the refrigerator is still quite usable for manual rework or small assemblies. You can get syringes of paste that fit whatever process requirements you have - leaded, lead free, hi-temp, no-clean, solvent clean, etc.
If I’m doing a whole PCB assembly of SMT I order a stencil from OSH Stencils, apply paste and place components, and do the board in my “Table-top Reflow Chamber” (also known as “Electric Skillet from the Second-Hand Store”).