With regard to FlyBe the problems seem to be linked to technical problems with ATR72 EI-REL and both Stobart and Flybe do not appear to have spare aircraft which is not a surprise given that it is the summer peak season.
I think issues with other carriers are weather related. BA CityFlyer and EasyJet have been impacted by the weather on the island on Saturday morning and particularly given the thunderstorms that have been prevalent in the South East of England over the weekend. This must have impacted on schedules and crewing hours issues.
At least EZY have operated additional rotations to/from both Gatwick and Liverpool today in order to catch up with some of the weekend cancellations.