Night trains are very popular and have limited capacity so most summer dates are sold out.
There are only seats available on 15th July between Zurich and Lesce Bled/Ljubljana : 2 x 13.20€ through tickets.oebb.at. Add Eurail as a discount and select one-way tickets
Alternatives :
- daytime train
- Nightjet to Graz : seats or couchettes in a lady compartment only
- 2 night trains to Vienna (through tickets.oebb.at) : seats, couchettes and sleepers available. Then train to Villach (departure 08:18) and Lesce Bled/Ljubljana -> second pass day needed (or you buy regular tickets)
In any case book night trains ASAP