Train isn´t available as it should be

Hey, I just created a whole tour through europe in the summer and now i want to make all the necessary reservations but im getting into big trouble whilst doing.

My train from Bruxelles-Midi to London St.Pancras on the 29th August from 17.21-19.00 says that the reservation is `temporarily unavailable´ and i thin it can´t be overbooked cause all the other trains that day do perfectly fine.

Also i tried getting the reservation via the offiial website for the eurostar but there this train doesnt even exist what makes it even worse.

This is not the only train with which i got trouble so i would really appreciate some help.

Thanks, Luca