In the picture you can see how clean now is both the PCB and the switches.

To assembly you often have to put the PCB into he bottom half, carefully threading the battery contacts into the case rails (if any), then aligning the buttons over the PCB and finish with the top half, be mindful of the LED or window in front, buttons should align themselves.

Is time to test again(like we first did) and if any button fais repeat the cleaning both in the PCB and the button itself with a cotton an alcohol. To ease this process you can test without the top halve, just aligning the buttons, or putting the top half but not pushing all the way until makes click "click" (more easily said than done)

If continues not working after you clearly see it clean you should pay attention to the graphite, there are some products to paint over them if they were worn out. Now clean and easy to see check for any lift or broken tracks.

If all goes well put the screw back, put some new batteries and pat you on the back, congratulations you made it! If all didn't go well still pat you on the back, you tried, next time you will not need a guide.