That'll happen if you run off the end of the connections that CD can book for you. For example I recently tried to book one all the way down to Trieste and it c

That'll happen if you run off the end of the connections that CD can book for you. For example I recently tried to book one all the way down to Trieste and it could show the trains but couldn't book them for me, you need to break it down into parts and book with relevant operators, in my case CD for the first part and Trenitalia for the last, skipping OeBB in the middle. Just beware when you do this that different operators will show different trains and connections so you may need to manually select the trains if (for example) CD and Trenitalia disagree over the best links. Seat 61 has an excellent guide on dealing with this, https://www.seat61.com/european-train-tickets-online.htm https://www.seat61.com/european-train-tickets-online.htm.