It is unlikely to be fully booked. There seem to be many problems with the Polish reservation system lately. I'd normally advise to book this train via CD but it also times out. To book via CD, you add the Interrail discount, look for the connection and click on "I want a reservation only”. Maybe it works if you try later; difficult to say.
Other than that, you can book at a ticket office that can do international reservations, also outside of Poland or Czechia.
PLUS that In PL REServ only open 30 days advance-for domestic-for OUTside its usually loner and this rfear for ´full’is really ridiculous so long advance.