We booked through an agent and I couldn't access seat reservation at all until they had ticketed the trips, then after a day or so the middle seats showed as bookable. But I was advised by QR that they reserved these for couples of families, so if they're not visible then it's just a matter of them ticking a box to say you are travelling together.
Myself and my partner aren't married, so the delay can be due to having different surnames.