I am trying to buy two tickets through [bahn.de](https://bahn.de) (tried both the Android app and the web site) for a train from Amsterdam to Frankfurt on 14 July departing at 8:08. The problem is that it offers me a choice of ticket options, but after I choose one and proceed, the site says that no seats are available and because a reservation is required, I can’t buy the ticket. That seems weird, as a) If the train is sold out, I wouldn’t think they would show a price, and b) I thought that trains didn’t sell out this far in advance.
If I try for the preceding or following days, there are seats available for the route I want, so there isn’t a general problem with the site. If I try for the schedule I want using another service (Trainline), it also fails in a weird way: it gets to the end of the selection process and then says that the fare is no longer available. The train at 6:38 offers seats, but the 10:38 one also has a weird error, where it says the seat map isn’t available yet but I can buy anyway (even though a reservation is required).
So,
1) Is there anything else I should try to see if this is truly sold out?
2) If the train is sold out, are they likely to add more cars? And if so, how long should I keep checking before I choose a different option?
Any advice is appreciated! I won’t be in Germany until I take this train, so I can’t try to buy in person, and I don’t speak German so I haven’t tried calling.