That's odd, as there was a lot of seating last time I got it (not sure if that's still the case though). However it is a 'normal' train that just happens to run at night and have sleeping accommodation, rather like the Scotrail sleeper between Edinburgh and Fort William (only), so unlike the other sleepers, reservations are not essential.
Perhaps reservations close for seats early? it's normally 2 hours before departure (I believe), or maybe some seats are left unreserved (like the buffet car on East Coast services on Coach B on XC)?