Standards differ

I want to know how is it possible that every time I want to book an exit seat I am told by the call center to do this at the airport but when I request an exit seat at the check in counter, they confirm that all the seats have already been booked - doesn't matter how soon I get to the airport. I get the same response. Please help me or give me directions on how to book an exit seat for any future flights. If everyone else can book exit seat, surely I can as well?<br> <br> Second query: I've booked an avios Economy flight because there was no more Premium Economy or Business flights available for Avios bookings... WHY then bother paying with Avios. This is a system that seems to be very ******* as there are always Avios bookings not available and even if there is a class available to pay with money, I cant upgrade my booking!