I would recommend to keep checking the website. It is just a request, not confirmed, until shortly before the cruise, so in the weeks leading up to it you may find some people switch their request to main dining and it shows up as an option. I was at first assigned to late dining, but then saw main show up in the drop down menu a few days later and snagged it. Also, don't be afraid to keep calling and asking, and check in with guest services on the ship -- from other threads I've seen, it seems like this is one of those "squeaky wheel gets the grease" situations.