If you just buy the normal fares from the train company websites then you may as well not have the passes at all as you are getting no benefit from them.

You will be best to post up which overnight trains you are intending as well as which type of accomodation; seats, shared couchettes, private sleepers, etc you are wishing to use.

Unfortunately there are different ways to get the interrail discounts and different routes have different levels of discount for passholders.

Some nighttrains indeed only have seats and no sleepers, mostly in Germany.

Some other will not sell sleeper easily to passholders who bring in very little money

Others simply do not show space that is all reserved and this may indeed mean that you are only shown the remaining bargain basement. Or they let you do it via another and rather vague way (see overveiw by seewulf for more info)

BTW-pass is for 1 month-start in the right month and it can last 31 days

Thanks for the replies, much appreciated.

I’d like to take the overnight sleeper from Salzburg to Zagreb on 10 July. Leaving at 1.40 and arriving at 8.32. I’d like a 3-bed sleeper cabin. We are two adults and a child. But on the EU rail site, there is no option to reserve a sleeper, just a seat. 

Coming back, I’d like to take overnight service from Bari to Turin on 30 July. Leaving at 22.10 arriving 9.25 the next day. Again, I’d like a three-bed sleeper cabin. Same problem, no option to buy more than a seat reservation, unless I go directly to the operator’s website. 

 

The advice from the experienced travellers in the community is to use other ways to make reservations than the Interrail/Eurail website. You can look at the guide in the link:

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

If you, after having looked at the guide, have questions about how to make specific reservation, please give your travel details (departure date, time and route) preferably in a new post, and you will get advice. 

Can be booked via ÖBB. I do see that sleepers are not available anymore for 3 people, only for 2. Couchettes are still available. If the child is under 6 and needs their own berth, enter an age of 6.

A reservation for this train can be booked by phone via Belgian railways. No booking fees, no pass cover number needed, reservation is sent via e-mail. I can see on the Trenitalia website there are still sleeper compartments available.