Assuming the obvious (serpentine belt is spinning the pulley for the P.S. pump), the first guess would be that the pump is no longer working adequately.
If you remove the reservoir cap, you can usually see some movement in the fluid with the engine running as the fluid circulates. Seeing that doesn't guarantee it's working right, but not seeing any is another sign it's likely not working. Should see more as the wheel is turned, and engine should slow slightly when wheel is first turned B4 computer adjusts to maintain idle speed.
P.S. pumps are relatively cheap, and a much more common failure point than the rack, so replacing one is usually the first step when P.S. isn't working - particularly if you don't see any of those signs of it's working.
"Rubber Side Down"
S.R. #275 Focus Takeover #003