Keys stop being responsive.

I have been using this keyboard for years and I really like it.

I only had to replace the battery two times.

Unfortunately some keys are gradually becoming less responsive, requiring more force to be applied to register a key press. Notably the 7,8,9 numbers on the numpad and the ~ key under the esc key.

I have seen that it can be opened but can it be fixed by simply cleaning the internals or something similar? Or are replacement parts of any kine required?