Could be heat damage from soldering. They may just need cleaning. A contact cleaner/lubricator something like this https://www.jaycar.com.au/contact-cl...y-can/p/NA1012. I use Servisol a lot (though it comes in different packaging in the UK).

Don't ever use WD40 for pots or contacts! It is designed as a water dispersant, not a cleaner, and leaves an oily residue which whilst initially making things better, attracts dust and quite quickly makes things a lot worse.