Hello MeganA
You have to be patient.
Right now, not even the final schedules for June are published, due to engineering works. It is very uncommon to book tickets that far in advance, yet absolutely sufficient to purchase them a month in advance.
This year, a lot of engineering work is planned on that route. Therefore, you have to expect fewer train options, cancellations, or timetable changes. Thus, your current planning for October is subject to changes anyway. If you need a reliable schedule now, travel Montreux to Milan via Domodossola - Locarno - Chiasso.
Best regards,
Roland