We will be in Rome mid to end June. Taking a day trip to Naples and Pompeii. When I go to book a train, early June has a train leaving Rome at 7:00am. But on June 21 the day we want to go, it appears the first train is not until 8:00am. Is this correct? Or is it too early to book and that schedule will be released soon? I'd really like to get to Naples as early as possible so we can get to Pompeii when it opens, explore, and still have time for Naples.

Train timetables change the second Saturdays of December and June.  Until the new timetables are released, depending upon where you are searching for tickets, you may get messages that the trains are full, that there are no trains, or (accurately) that the new timetables aren't available yet.  The June table is not out, so you'll have trouble booking anything on or after Saturday, June 10th right now.  Another member here said they called the railway and were told that the new timetables should be out by the end of this month.

We took a tour from Rome to Pompeii and stopped in Naples to look at the view.  Its a long day, but worth it.  I looked the train as well, but it didn't see to work out timewise.