The routes in question are noticeably seasonal and if they had high loads in summer (possibly certain days only) this could not be expected to continue through the winter. I remember returning from Waterford to Luton with Euroceltic one winter afternoon with about 10 pax in the F27.
It certainly seems poor though to mess about with schedules like this so they do not become established. Aer Arann used to lease aircraft from VLM (and probably others) to cover downtime on their fleet.