Seat Allocation

I am flying with virgin to Las Vegas from Gatwick in April in economy, the aircraft is a 747-400, now on virgins website when they give you the map to pick your seat i have chosen seats 45A & 45B now seat 45C on the virgin map shows as being taken but if you go to expedia and other websites on their seat maps for the same flight seat 45C is actually not available at all just 45A,B appear and 45C is missing. this is also happening for some middle row seats as well where they don't show up at all they seem to be blocked. Can anyone explain? I am travelling with my dad and would prefer that no-one sits in 45C.