Hi
It could be that they need cleaned and also need some new carbon applied!
Iv seen keyboards that look new on the inside where their pads and the keys are and all they need in some new carbon
To apply it you can just use a normal pencil and simply draw the new carbon on but don’t put too much on and only put it in the pads and not on the circuit boards as we don’t want to short anything out!
Hopefully this helps
Any questions please ask
Thanks:-)