Solder joint only working while solder is still liquid
If all the caps are showing the same problem, my first guess was the potentiometer is broken. You may have broken it by your inept soldering attempts. It's a mechanical device so it's prone to heat-related problems. The caps aren't.

For the record, how to solder:


remove all the old solder with a soldering iron and solder wick (beginners don't use a pump, and pros seldom do so.)
clean your iron on a wet —wet, not humid— cotton cloth
apply the soldering iron to all the pieces you want to connect for two seconds
apply the solder wire
remove the soldering iron
remove the solder wire


The solder joint must be shiny. If it is dull, you made a mistake. Most likely you moved the parts while the solder was setting. Repeat the whole process. Do not attempt to smear it with more solder.

Oh, and don't use lead-free solder. It's a nuisance for hobbyists. Buy SnPb38Cu2 or SnPb38Ag2. They are very easy to handle. Some SnBiAg solders also are, but much more expensive. Don't buy SnPb40, it's usually without internal flux core, which makes it unfitting for electronics. Buy 0.5mm solder wire. It's easier to handle than thicker ones.