Possibly the service is ''unreserved''. For some reason the booking sites class this as unavailable (yet every seat is available) and you cannot book and proceed to payment. It means sites like trainsplit can't work out a split for services you want as they too show ''unavailable'.
I've had to resort to buying an off-peak return for an early service just to get the ticket, Then had to go on twitter to get a seat reserved, which was utterly pointless as all seats were available anyway.
Avanti have lost 3 bookings from me this year already as I genuinely believed the train was fully booked and wouldn't have got on if I'd chanced it.