If you are looking at dates after March 2015 the reason will be that flybe have not released these flights yet, only when they do you can then book them via BA website. Keep checking their website and try doing dummy internal Scottish bookings and you can see when dates have changed.^

Edit
Just checked the Flybe ADS site and internal flights are up until Sep 2015 but i am getting the same error message when trying to book through BA Website from KOI-LHR which is unusual if available from Flybe.