I'll be travelling from Finland to Italy through Zürich in December. I have booked the ÖBB NighJet train from Hamburg to Zürich and plan to continue with an EuroCity train from Zürich to Milan. This will be happening in mid December.
However I am unable to book tickets for the train from Zürich to Milan due to some error. The train also does not show up on Trenitalia's website. On your International travel FAQ you mention there being some timetable updates during December but I would like to make sure this is the reason and I would also like to know when I will be able to book these tickets.