I am not sure if my recent experiences trying to book a couple of reservations, well in advance, across Italy are repeated elsewhere but here is my experience.

We need to get from Monaco to Venice in September. Trenitalia shows a journey at 0901 with changes at Ventimiglia and Milan, but there seems to no way to book reservations only on their site. Neither OBB or Interrail planner showed this option until today, when the Interrail planner/reservation tool now showed the combination we  wanted. Reservations from VM to Milan and Milan to Venice would be needed. 

OK so far - moved to reservation tool and proceeded to make both reservations. Went to check the cart, entered passenger details and it said one of the legs wasn’t available (The 3 euro intercity from VM). Really???? A basic Intercity 2 months in advance.

Back to the planning phase.

Now, with Monaco to Venice as search,  it seemed that the VM to Milan had disappeared and the suggested option included 3 changes to Milan.

Then tried simply VM to Milan in the search. Lo and behold the unavailable train was now there. Quickly added to my reservation cart with the pending Milan-Venice reservation and now no problems. We have carriage and seat details, paid and delivered via e-mail in minutes.

So it seems true that try, try again is the way to get through this stage of holiday planning.