Doesnt take 10 minutes to put a few seat reservations out im sure. They often start putting the seat reservations out when the terminating train is reaching its destination ready for the train journey back. Sorry, if they offer seat reservations then they really should put them out IMO▸ , otherwise dont offer them at all.
Passing through a train with several hundred reservations all on a little roll of paper can take an awful long amount of time. Seat reservations can only be printed at a station with a suitable terminal, for my area (
SWT▸ ) that is only really Weymouth, Exeter, Bournemouth and London Waterloo. If the demand is high enough, the printer is broken OR there are networking difficulties with the National Reservation System/Service then no reservations can be printed for certain trains obviously.
If seat reservations are not offered on certain services, then advance purchase is likewise not available on said services. Put it this way, seat reservations are issued so that no more than the trains booked capacity is issued. Therefore all passengers with a reservation should be able to obtain a seat except in the possibility where it is a busy train. Either way, if they're not on display then no reservations apply.