Try this - bind unused keys to the windspeed controls. This will verify that the function works. It is temporary, and will be lost the next time you restart RealFlight.
Make sure Wind Gust, Turbulence, Wind Variation are all set to "0".
Go to any airport, any plane, fully ready to apply throttle and takeoff.
Hit the tilde key ("~"). In the command line that appears at the bottom of the screen, type this, exactly as shown:
bind b windSpeed 1+
Hit Enter - you should get a confirmation that the bind was successful. Then type:
bind g windSpeed 1-
Again hit Enter, and see the successful confirmation.
On the far right of the title bar that says "Console" is an "X" that allows you to close the black command box.
Each time you press "b" the wind speed should increase by 1 mph, and you get the green confirmation text shown above. Pressing "g" will lower the speed by 1 mph, down to 0.
If it works, then the function is operating correctly, and your computer keyboard is not passing the PgUp/PgDn key presses as most normally do. If it doesn't work, then something much deeper is wrong.
If you can't find information from your PC maker about the PgUp/PgDn issue, then you can make the bindings permanent. I picked "b" and "g" because they were some of the few keys that RF doesn't use by default for something. If you want the permanent solution, let us know, and we can suggest more sensible keys to use, although that may mean either losing two of the standard ones RF uses for other things, or doing some kind of combination of two keys presses, at the same time, for each. Yes, they can be modified, so the speed increments in bigger steps - one press gives +5 or +10 mph, etc.