Seats Available with VA

My forthcoming trip is booked through Virgin Holidays. We fly UC out, but Premium Back. I have now asked if there was any availibilty to upgrade to UC. Virgin Holidays have now come back to say that there are no seats available through them and that the have asked their flight department to request more seats from VA, but there are no seats to request. I have been on to VA website and there are still seats available for the inward bound flight, how come flight department are saying that there are no available and to ask at check in in case there is a cancellation? I am very confussled.