Which date(s) you’re trying to book, at what time do you want to book?

And which website do you use? Interrail/Eurail or the b-europe one ( https://www.b-europe.com/EN/Booking/Pass#TravelWish )?

Hi, I tried all reservation between today till 15 December, nothing works. I tried multiple times between those dates. It shows me that it is available then suddenly changed  to sear reservation unavailable. It happened to all dates I mentioned above. 

I used Eurail.com. I cannot use b-europe because I have only digital eurail pass (why b-europe reservation needs physical eurail cover pass number??)

You can use it! By using this tool:

There are no Eurostars between London and Brussels on November 11th and a reduced service that weekend, due to engineering works.

PLUS that this €* had a kind of big sale the last days-its site may simply have been overburdend

If B-europe.com shows availability and eurail.com doesn't, then it's unlikely to be a problem on the Eurostar side.

I have often noticed the IR site doesn’t show availability for E* whilst B-europe has plenty of options. Now I always first use B-europe for availability for pass users.