There were a couple of co-pilots who were never made captains inspite of flying for many 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 junior captain.
They were contended to be just senior co-pilots because of lesser responsibilities as compared to a captain
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.
Singapore Captain explains:
The reason being in the past, First Officers do not need a ATPL (Airline Transport Pilot License), and to qualify as a Capt, u need ATPL, but after flying for so many years, these FOs brains are no longer that active and they find it difficult to pass the ATPL exams. Now, all FOs have to take their ATPL exams during their cadet flight training days.
But then, recently, there seems to be a higher number of FOs who failed command training. 5 FOs in the previous course failed their training, which is higher than normal. To be Capt, u need to pass this command training which is highly stressful. Some say that before starting command training, the FO should buy a pet, and vent the frustration on the pet after he reach home.