Do you have any type of multimedia keyboard? On my UMAX MMKeybd I cant use F-keys in LFS properly when I have multimedia function enabled. I must switch off the multimedia functions on KB by special button and then it works. It might be similar problem in your case.
Check your keyboard settings (even if you're not using keyboard). If anything is assigned to the arrow keys for the keyboard controller you will need to hold shift + left/right to change the pitstop settings.
Go to options > controls and see if any bind uses "left arrow" or "right arrow". Remove those binds by selecting them and pressing DEL. After that everything works again.