It happens often that interrail.eu gives to 2nd class users 1st class (or business). You are not allowed to use 1st class with an 2nd class pass. You have to get in touch with customer service of Interrail here: Submit a request – Eurail Knowledge Base 

If they do not respond before your travel, ask directly in the train stations at the ticket office of Trenitalia or in big stations at the last minute Freccie Kiosk for help. Also possible to ask BEFORE BOARDING the train staff. 

We in the community highly reccomend to NOT use interrail.eu reservation service in Italys daytrains, due to this problems and do book directly at Trenitalia Ticket Offices, SNCB/NMBS (maybe also NS International), DB, SBB CFF FFS or ÖBB. Online Possible via ÖBB using this steps: How to get Reservations | Community (eurail.com)

Hi,

I wonder how you got this fixed because i have the same problem and on the OBB website i cant make seat reservation. I try to email eurail, trenitalia, the chat on trenitalia but no help anywere. 

Kinds regards
Tania

Hey Tania. I actually used the OBB site to make the new reservations. You should choose the Interrail option as a discount card, and choose the option to buy a ticket (not seat reservation only). This sounds weird, but it works! I will try to get my money back from interrail later for the mistake they made.

Hi,

Thank you for your reply, the problem is i did that and it is not possible to make a seat reservation for that train on the OBB site. 

Tania

Then I think you should contact the NMBS or NS international. They can also make international reservations.

Yes you can book also via SNCB/NMBS Ticket office or call center. 

Wich train do you like to book and when? So maybe I can tell you why it is not working. 

I wanted to book from milano centrale to genova piazza principe intercity 669 14.05 on 6/8 . I was thinking as well to call brussel office on monday i just hoped i could fixed it myself.

thanks for the help.

Tania

I can book it for 3€. The EC at 14:11 has a problem via ÖBB, and can’t be booked, but it would also cost 10€ for the same time like the IC. At 14:25 there is the local express train RV to Genova, which is reservation free (but on weekends, trains can be full).

I dont? But yours is on the 5th en mine on the 6th

Yes sorry, but for 06/08 I can still see a price. 

The strange thing is, that I can’t see a price for the IC at 15:10, but you can. This is really strange. 

Maybe try to book with the website in German? Maybe than will it work?

Hi,
Nope i try it on German site, i try on my iphone, ipad, PC and on mac book, and all different kind of browsers from edge, safari to chrome i get the same result, nothing :-( 

So weird.

@Tania Baeyens I think I know now what it is missing. The Interrail discount for your child. Did you add it? I tried now, and choosing adult (Interrail)+ Child no price showed. 

You can also use 2 times adult. You need a seat, the cost is the same if child or not.

I tried an adult discount globalpass + child discount globalpass , i tried two adults discount global pass, i tried without discount it al stay the same it is not visible . 

strange, I hope SNCB/NMBS can do it. There should be enough seats available. 

Thanks for helping.

I called brussels service center en they helped me out in 10 min. I really recommend the service center they are really nice people always ready to help! 

Yep, they're normally quite good. The only downside is the cost of their phone number (€0.30/min.) so I tend to go to the ticket office if I can.