There is a limited number of pass holder seats on the Eurostar between London and mainland Europe. 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

If you don't get any result, 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

What is your travel date ? We can help you with alternatives. For example if London - Paris is full, London - Lille has way more seats.

But it's true that Eurostar has a limited quota which can sell out weeks in advance. If you have been planning for so long, you should have bought the seat reservations well in advance. Eurostar is a true bottleneck.

If you let us know your travel dates, we'll gladly help you.

I had no idea the trains would fill up like that, first time I have ever used the train in Europe. But i kept checking and something opened up for our dates so I was able to book. Thank goodness!!

I'm glad !

The Eurostar is a bottleneck and you're travelling during a busy period (bank holidays) + it's soon summer. So yeah Eurostars definitely sell out. The quota in place (on Thalys trains too) means that even though 250€ tickets are available, there's no passholders seats left.

There used to be more trains but with Brexit it's now impossible… it's not that Eurostar doesn't want to put more trains (right now)

Don't worry, it won't be as stressful in the rest of Europe (if you're planning to visit)

And to add; for a lot of trains there are reservation free alternatives (or alternatives with less pricey reservation costs). Only Eurostar is hard to avoid...

Best examples are the horribly expensive Thalys and TGV Lyria, it’s very easy to dodge those trains or limit seat reservation cost. Just give a shout here if you want advice, and don’t forget to mention your route, travel date and hour.