Do you have a flight in particular you are looking at? I seriously doubt it has anything to do with cookies but if you are truly paranoid you could try booking in an incognito browser window. CVG-SEA does not operate Tuesdays and Saturdays in May, nor on Sunday May 29th (the May schedule was reduced to less than daily several weekends ago). Google Flights is showing coach fares available on all flights in May, although there is only M bucket currently open on May 27th.