Welcome to FT gapmichael

I can't think of an obvious example of where that should happen. I guess the seats are showing as not available when you view the seat plan on the BA site? There are some restricted seats but they usually relate to status rather than type of booking, for example 1A/1K in long haul first are not selectable unless you are a BA gold or oneworld equivalent.

Are you able to share any details, such as what flight you are looking at?