What is the route, date and departure time ? There could be multiple reasons : train sold out, train unavailable to book on eurail.com,…
Btw it's always best to avoid eurail.com to book seat reservations because they add a 2$ booking fee per person per train. General advice : https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105
Really give us more info and we'll be happy to help you
And btw what do you mean by downloaded the tickets ? If you've bought regular tickets, then you don't need a pass
London St. Pancras Int-Bruges 11:04-15:30, Tuesday June 6th.
ES 9126 London-Bruxelles-Midi, 11:04-14:05
IC 2835 Bruxelles-Midi-Bruges, 14:29-15:30
For Eurostar (and Thalys) check availability here : https://www.b-europe.com/EN/Booking/Pass#TravelWish
You can also book it using the above link : 4€ booking fee per order (no matter the number of trains or people), compared to the 2€ per person per train on eurail.com
As you can see there are 30€ seat reservations available. Book ASAP though, they can sell out weeks in advance
Brussels - Bruges : no reservation possible on all trains in Belgium or the Netherlands (except Thalys which should be avoided). Before boarding simply add the train you'd like to take to the app and find a free seat :)