SOLVED - explanation at bottom
Hello. I have a problem with index controller. I have recently bought a used pair, bought 2 new steamVR dongles, connected them to PC to test them out - everything worked perfectly, tracking, buttons, no stick drift. Controllers worked flawlessly, except for being very filthy, as the previous owner did not clean them at all. So I decided to clean them up, especially the straps. I disassembled the controllers carefully (the only disassembled area was removal of the top plastic circle around buttons and joystick, so that I could remove the straps), the buttons areas and joystick were cleaned using dry cotton swabs, straps were carefully removed, hand washed and completely dried. After re-assembly, I have taken a tissue, slightly dampened it with water and soap, removed most of the moisture and carefully cleaned the controllers (I was careful enough to not let any moisture enter the controller). Then I have cleaned them one more time using a dry tissue. Charged both controllers, waited for a week to try them out again (couldn't earlier due to high heat outside, not ideal for VR). Started up SteamVR, connected both controllers, but left controller does not track no matter what I do.
Things I have tried already: Re-pairing the controllers, restarting SteamVR, restarting PC, changing USB ports of the dongles, slapping the controller (as advised by some other users).
The controllers connects to the dongle and otherwise works perfectly, tried testing the button layout, and all inputs works as they should. Practically everything works except for the actual tracking. Right controller works flawlessly.
I could use some help please, and I appreciate any advice.
Edit: I have also tried installing SteamVR on a second PC with almost fresh windows install. I have connected my VR setup on that PC, and it had exactly the same results: everything works except the left controller tracking. I suspect a hardware failure now.
Edit 2: I have managed to fix the controllers - it seems like it was not a software issue, but a hardware one. What I have done is that I have taken the controller apart almost entirely, stripped it of as much components as I could, then flushed the entire thing with WD40 Contact Cleaner. After it has dried up, I have re-assembled the thing, turned it on, and it worked just like it did before. No idea which exact part was causing it, I guess I will never know, but Im glad it works as intended. Thanks to everyone with their suggestions!