Are you using "auto-takeoff"? It tells you what to to.
If you are trying to take off manually, then use <SHIFT><+> (the plus sign on the keyboard, not the keypad) to go to full throttle.
Do not use the RPM commands, thinking they are the throttles.
You can see and test what is mapped to your keyboard in the Options->Controls->Map Keys panel. If you are using a non-standard keyboard, you may have to use the "Fix Keys" button (it tells you what to do) in the Options->Controls->Map Keys panel to fix the key so the game will recognize it. That is only if you press it and the virtual keyboard does not show it being pressed.