This is really hard to Diagnose over the net. I've never opened any of mines, so I'm not really sure about the specific mechanic/hardware behind the buttons. Judging by pictures that can be found on the internet there's not a lot you can do (also nothing to solder as mentioned above). If there's some rubber pad creating the contact (pushing contacts), this might be the actual issue (in case it's somehow squeezed too hard, too brittle, etc.). You might be able to fix this with a small piece of paper or cardboard being added between rubber and button, but it's also possible the culprit is sitting somewhere else.
Not the most environment friendly solution, but consider how long it takes you to find the actual issue and fix it compared to the price of a new controller. If you want to, you might be able to replace the whole button with all parts involved with those from one of the other controllers (are those broken as well?), but ensure you're not mixing different hardware revisions in case there are differences (don't expect any).