Sometimes reservations are not possible if the consist that is going to operate the train has not been finalised yet. How far in the future is this trip?

My advise is to not bother. Get a reservation at the station once you are here. Or just board the train and find a set of seats together that are not occupied. Because 1st class tickets come with an automatic reservation there are always lots of reserved, but unclaimed seats, as many travellers do not travel on the train they originally selected when buying a ticket.