From the renegade forum, It’s not a common problem but some have suggested:
Take your foot off the throttle and then push the "set speed" button. The pedal is "drive by wire". And for simplicity, cruise is the same. You are trying to send two signals at the same time: the pedal will always win.
Or
Dodgy clutch and/or brake pedal switches can be the culprit for CC not engaging
Or…as above
Normally faulty or jammed brake (and/or clutch) pedal switches are the main reason
for cruise control not engaging or disengaging intermittently