Trust the Eurostar website and book pass holder seats through b-europe instead of through Eurail, see below. 

There is a limited number of pass holder seats on the Eurostar and on popular departures and during high season those sell out weeks, and sometimes months in advance.

The best place to see the availability of passholder seats and make reservations on the Eurostar is 

https://www.b-europe.com/EN/Booking/Pass#TravelWish

Press "later trains" and eventually you will find the next available connection. 

If you have a mobile pass you need to generate a Pass Cover Number in order to make the reservation at b-europe. You do that here in the PCN generator:

https://community.eurail.com/news-and-announcements-39/pass-cover-number-generator-is-live-5653

Thanks for that. Much appreciated.

Never trust the Rail Planner App. It only updates monthly. Always use it in combination with the Apps or websites of the railway companies you will be using on your trip. Then you will have all the latest, most accurate rail time information (there's always a risk of engineering works or incidents causing delays or detours).

Most, if not all, railway companies have good English versions of their apps.

Some rail planners are quite usable outside the country too, like DB Navigator, ÖBB Scotty, B-Europe, SBB,...