If it's not offered (and in my experience it is 80% of the time) I always ask. If they say they have no better rooms available I go to the room given and check online if they are still selling rooms at the next level, and if they are I go back to the desk and complain. Never fails, platinum upgrades are a right on a paid stay if they have rooms available, not discretionary at the whim of whoever is on the desk.