Having gone into the terminal a number of times for a coffee before flying at the club, it has become increasingly apparent that the number of passengers appears to be very low - even with a flight due to depart within an hour. Speaking to some people who have lived in the south west all their lives, they were not even aware that Plymouth had an airport (could be ignorance, however it has happened a couple of times).
It must be very difficult for a small, local airline in an area with a relatively small population to make it's mark. Plymouth is too far from London to run an effective local commuter flight every morning and afternoon, and for the holiday makers, why not just get on a train to Exeter or Bristol and fly from there? Also, the members of the armed forces staying there will have forces railcards (students will have student railcards) making the railways more tempting to more people. Put simply, the demand is just no longer there.