Try the ÖBB website. Do NOT choose seat reservations only but put Interrail/Global Pass in 1x Adult ...you'll see what I mean at the website.

Worked for me booking a train in Italy so I hope it will for you too

Thanks for the fast answer.

Already tried this one, but the tickets aren’t available.

Ok, bit did you choose the ticket option ? 

I made the same mistake and also thought that there are no tickets available...

Hmm yes, did choose the option…

Thanks anyway

As so often: not too late (why do even the swiss think so?) but too early. The old standing rule is/was: from 2 month before.

Try an earlier date to see if my theory holds, I cannot know everything, but the main source of train newbees here on this forum (but then I always think the schwyzerli kennen ihre bahnli) is always far too early and die of fear that their prepared plans fall through the roof.

Plus that TrenItalia has indeed a habit of only releasing data/timings at the very last moment.

BTW: it should simply also be possible to stop by at an empty moment at SBB and have them arrange it for you……………..Italia ist ja bevorzügtes bahnlibesuchli land fuer CH

thanks for sharing your wise experience, was helpful 

Just split up the booking. Multiple trains in one go doesn't always work. You didn't specify the complete journey but the screenshots showed that only the first 2 trains need a reservation: