This has been an ongoing issue. There are apparently two places the seat assignment is stored, and the one you see when you pull up your reservation can apparently be out of sync with the "real" one.

One way to check is if you do a dummy booking, you can normally see which seats are really available (i.e. your new selection should appear as occupied)...

Or, for that matter, when you pull up the seatmap again, you'll notice that while you appear to be in the same seat, the seat you clicked on is mysteriously now booked...

Dan