In general, one need to wait until the following are all available:
- reserved seats for each train on the journey for which seat reservations are provided, and
- reserved places for each train on the journey for which seat reservations are not provided but reserved places are provided.
An example may help.
At the time of writing (1410 on 24 March), ATW reserved places for Advance tickets are available until 15 June and TPE reserved seats for Advance tickets are available until 17 June. If I wanted to buy an Advance ticket Leeds - Cardiff via Manchester for 16 or 17 June, I would have the choice of:
- buying the cheapest Leeds - Manchester ticket now and waiting until Manchester - Cardiff tickets become available, or
- waiting until through tickets become available (which may or may not be at the cheapest level because the cheapest Leeds - Manchester reservable tickets may or may not have sold out).
There is no guarantee that one method will give a cheaper overall price than the other.
Sometimes there is a work-round. For example, if the cheapest Leeds - Manchester Advance tickets sold out, I can sometimes get the cheapest Advance ticket Leeds - Cardiff via Manchester by asking the booking engine for fares "via Hebden Bridge". This avoids the reservable TPE services.