For the most trains you are looking to early and the question is for which trains are you looking? Nighttrains or daytrains? Nighttrains are possible up to 6 months in advance but often just 60days in advance. Daytrains it´s usually 90 or 60days in advance.

Prague - Budapest can be reserved directly on cd.cz
Budapest - Zürich via oebb.at (here take notice of the 2 different manuals for daytrains and Nighttrains)
 

Way too ealry-untill very recent 2 month advance was the norm. PLus that smart travellers know many ways to avoid the costly REServ-though this may not apply if you go 1st.

BUT; profile says youre from UK-and EUrail is NOT for Europeans (common brit failure: it has really nothing to do with EU-so Brexit did not change a thing in this. You´d better get this rectified asap. Unless you mistakenly took this for USA-or have a passpt from out of Europe. EURopeans can only do 1 trip OUT and at end 1 trip back IN in home country-that has to be stated. For rather fairly obvious reasons.The pass is INTERrail.

Thanks for the replies….so, for example.  I want to book a night train on October 8th from Budapest to Zurich.  The train is the 20:55 arriving Zurich 08:20 on the 9th.  On eurail and OEBB both say ticket unavailable….??

You can already reserve this train on oebb.at  please follow the instruction in this post (don´t use the seat only option for Nighttrains )

Thank you!...so confusing as it seems logical to press the “seat reservation only” button, 

Yeah because of that we created the post with the manuals :)