Ive had this issue before, started at first going on and off intermittently, then one day it totally packed it and wouldn't set anymore. After replacing the clutch pedal assembly inside the cabin of the car, its come back on and is working fine.

Might of been that the pedal may have warped, or loosened causing the computer to think that the clutch pedal isn't fully disengaged, thus stopping cruise control from activating.