Are you looking for train later than September? The train company Trenitalia that runs the trains in Italy has to update the timetable, it can be tomorrow or in some weeks. 

UPDATE: I see now that on the website of Trenitalia.com some trains has been updated. Look there. The railplanner app will be updatet in some weeks with the new data. 

This is for trains for a few days time, does this mean there are no seats left/ I am too late?

Don't plan in the rail planner app. It is often out of date and/or incomplete. For Italy, the best is to use Trenitalia.com to plan. By the time you travel, the rail planner app will have been updated and contain most or all trains.