There is a limited numer of seats for Eural/Interrail passholders on Eurostar. Once all those seats have been sold out, there are only normal tickets left to buy. 

Popular departures on the Eurostar sell out weeks in advance for passholders. 

On the 23rd of June there are only passholder seats left on 2 trains in the afternoon.

Here you can check the availability of passholder seats, and if you have your Pass Cover Number you can make your reservation.

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

Oh that is terrible. Before I bought the pass it looked like they had alot of seats left.  Thank you for your help.  Do seats ever open up on the train?  Can I make a seat reservation in the afternoon and then show up at the train station before the first train and see if they have any seats?

So three out of the four trains I wanted on my 4 day pass do not have any reservations available.  I just bought my pass today.  Any way of getting a refund? 

You need to check the refund policy for your pass. 

https://www.eurail.com/en/eurail-passes/ordering-info/refund-exchange-policy

Probably too late now but from Amsterdam to London, always also check availability in the Eurostar Brussels - London. Availability is much better there than from Amsterdam. You can use the reservation-free intercity train to connect from Amsterdam.