SEN lacks "critical mass" as an airport with insufficient destinations and operators to attract consistent growth following the initial surge. A runway too short to accommodate the leading short/medium range airliner in service will always be a handicap.
However SEN does have the twin advantages of a location fairly close to London and a direct train service from within 100 metres of the terminal building. The reasonable success of the Maribor route shows that there is potential for niche routes (Maribor is a really narrow niche!), and the Mediterranean destinations remain popular.
Surely Eastern European and Scandinavian destinations are worth trying....? Failure to grow to the 2 million target by 2018 will surely mess up Stobart's business plan though, so they need to get that fourth EZY Airbus back and attract two or three new operators (including at least one significant carrier) fairly soon, I'd have thought, or a profitable future will elude them.