Challenger Ultimate - 4 buttons do not work

[Kingprizrakus]

Hi guys ! I have a problem, 4 buttons of my Challenger Ultimate keyboard are not working:

F12, PrtScn, ScrLK, Pause :/

These buttons are located close to each other and have never experienced any liquid / chemical / physical harmful influence.

 

What I have tried:

    1. Removed the buttons and tried carefully pressing their membranes

    2. Manually disassembled a keyboard and checked that two films with electrical lines and contacts - are correctly located

    3. Reset all the keyboard internal profiles

Unfortunately, these steps were not enough to fix these buttons (meanwhile, all the other buttons are working)

 

Please tell, what other things I could try - to fix these 4 buttons?

(replacing a keyboard is not possible in my case)