If you provide more details you will get a better answer, it is hard to know what your problem is without the specifics.

Breaking down journeys into individual segments often works but In general not seeing specific trains is usually due to differences in how the booking engines prioritise services, overtaken trains will often be difficult to find on some sites.

Obb and interrail/eurail are the only online booking sites that will process Italian reservations.

Mabuhay! You are still too EARly-yes, I know it says from 2 month advance, BUT on 11/12 is the big yearly timetable change for al of EUR and TrenItalia is notoriously slow always to adapt this. Plus then the 2nd step has to come-OeBB has to load it itself into its system. As there are multiple trains-often even more as 1/hr, do not worry-even on the day you will find spare seats. What AlG here says does not make sense for this trip. You may check trenitalia site of there are already timings shown there-just for info.

DO however note you can go by direct train-mostly IC-along the coast OR via regional train to Firenze and then Hi-speed Frecce-cost a lot more and will barely save time

A quick check on Italiarail site (Trenitalia’s sales partner) is showing tickets at retail from 17 euros on the Frecciaargento. Plenty of choice of trains and 80% refundable up to 2 days before. A reservation for a pass is 10 euro. First Class on same deal is 0nly 21 euro and for fully flexible is only 66 euro,