Can't really blame the marketting of the route, as the Airport team and United have been working really hard on that aspect of things, I'd guess that NCL is losing out on folk within the catchment travelling via road/rail to MAN/EDI and GLA still. Try and pull those folk back in and perhaps the flights would be busier.
Mind you, with only 2 flights a week to Las Palmas in the summer, perhaps it's no wonder NYC may not be working...