The Eurostar website can't book any pass holder seats, so are you sure you booked pass reservations instead of regular tickets?

The Eurail website isn't very good and reliable for timetables or reservations; they also have an extra booking fee. so it's often more expensive than booking elsewhere.

For reservations, please have a look at:
https://www.seat61.com/interrail-and-eurail-reservations.htm

If that doesn't work for you, then please mention route, date and departure time for each train you'd like to book.

TGVs and Eurostar can generally be booked on www.raileurope.com without extra booking fees.

Spain is often a pain. Only some reservations can be booked online and then only via eurail.com. Also see this list of ways to get Spanish domestic reservations:

Hi Railmaster, thank you for replying so quickly. 

We have booked the Eurostar tickets on the Eurostar website.

The issue was with Eurorail, being two fold, linking our two passes when booking seat reservations. We have worked that one out.  We realised that we can only reserve seats a 72 h ahead at rail stations in Spain.  At least we know.

I will check out your links for the trip from Spain to the Netherlands via France and Belgium especially if we can avoid paying extra fees.

Eurorail support replied to my query about trying to reserve seats for two people in Spain and linking the passes and whether we had to activate them.   The link https://www.interrail.eu/en/plan-your-trip/tips-and-tricks/trains-europe/trains-country/trains-spain had useful information regarding Spain.

I have copied and pasted Eurorail support team reply:

“you do not need to activate your passes to book, the only time at which you need to activate is before your journey starts.
 
The second thing to keep in mind that Spain is quite a peculiar country when it comes to reservations - some of the trains (most relevantly, Regional trains and AVE international) cannot be booked online and still have to be bought in person. More information can be found here

As per making reservations for multiple people at a time. They can be done via our website. In order to arrange for that, you need to add multiple people to your website trip in the "trips and travelers" section (which can be accessed via the small person icon in the top part of the screen), If you still require more details, the process is explained in detail here. Remember that the system will prioritize seating you together if booking that way, but this cannot be guaranteed - in a rare scenario most of the seats are already taken and there are no seats next to each other, your bookings might still be divided.”

Thank you again for your help. 

Best regards

Michelle

If you have booked tickets at the Eurostar website you have most likely booked normal tickets and not Eurail pass holder seat reservations. How much did you pay per person?

We booked on Eurostar and had to quote our europass details. The tickets cost €76 and booking fee was €4 i.e. €80 total for 2 adults (standard premier).  Fare was 2 x Adult with fare Inter-Rail SP. 

It sounds like the Eurail website, not the Eurostar website? Anyway, the piece sounds correct for Standard Premier.

Sounds like B-europe website ("had to quote our europass details”), since it also has a €4 booking fee and has "EUROSTAR RESERVATION FOR PASS HOLDERS” as the title. €38 is the correct pass holder fare in any case. Regular tickets are more expensive.