Had a similar issue with our Cougar. Several members here advised me to look underneath the slide, on the outboard side of the frame rail, for a bell shaped stop. Loosen the jam-nut and then loosen the bell shaped stop. This should relieve the pressure that is preventing the clutch mechanism (that's the ratcheting sound you hear) to engage and allow the slide to extend. After you again retract the slide, you can re-adjust the bell stop.
Hope this helps
FWIW - If your unit is under warranty, you might have the dealer check it out.