1-Don't they have waiting list and priority on waiting list for Diamonds in this property ?
2-Who is clearing the waiting list ?
3-Quite agree that conventions, large meetings are not extra-ordinary demands but planned events and 2/3 rooms can be reserved for Diamonds until check in time.
4-I just wonder with the Hyatt Visa Card awards in suites, if that could be part of the reason - now that they are allocated before check in time, the hotel might not have any 'back up rooms/suites" for diamonds.