There's the yearly timetable change on dec 11th. The routes may not be uploaded yet on the planners (checked OBB Nightjet, found no tickets). Some companies are always rather late with uploading their itineraries. Better check every now and then over the next days/weeks, they should probably come up soon.

Maybe some other community member might know when they're available for booking. 

Milan - Vienna is not showing the train past Dec 10 which is going to be down to the timetable change, I can only assume some issue with Italian timetables.

Vienna - Amsterdam shows no availability on 29, 30,

Seats only on 28, 27,

Seats and couchettes on 31 (Happy new year on an overnight train not so popular I guess)

That will be down to the rest having sold out.

If you are not already, the best place to check/book is direct from OBB Nightjet https://www.nightjet.com/en/ticket-buchen

additional: the Milano stop will change to Rogoredo, as apparently the train gets lengthened to Genua from 11/12. But for the rest it is indeed as Al explains.