Recently I found out about the bus company Comati PSG which runs buses all the way from Sweden to Romania and makes intermediate stops in various countries between those. However, looking at their website, I can't seem to figure out how to actually book any route. I can select the departure stop where the list is properly populated with all kinds of destinations, but not the destination stop where the list only has the options "TO" and "Choose departure town".
I can also book journeys with them through FlixBus' booking interface, but they don't show all itineraries — if I search for my particular preferred one (Stockholm to Brno) I only get some FlixBus routes with two transfers. This is despite that if I choose a longer route (Stockholm to anywhere between Arad and Bucharest), Comati does show up and I can clearly see that my desired stop is in the list of stops on the FlixBus website (note that you have to choose a Monday or Friday).
With this in mind, what ways are there to book journeys with Comati PSG that allow selecting any given journey with them?