There were a couple of co-pilots who were never made captains inspite of flying for donkey years (some flown for 30 plus years). They either lacked the skills or kept failing their pilot exams or did not have the courage to take over the command in the cockpit.
Most retired as senior first officers perhaps drawing slightly (salary) lesser than a captain.They were contended to be just senior co-pilots because of lesser responsibilties as compared to a capt.
On a lighter note I am amused to read about a Singaporean pilot who does not mind flying for peanuts...read here. ( http://www.askcaptainlim.com/ is an excellent website if one wants to know about the job of a pilot or just for knowledge). Go and visit Capt Lim's website cos it makes good reading but no juicy stories..hahaha.