Having a bit of a further dig around, it looks like the issue may be that you are now at less than 28 days before your departure date. To get the prices that you want, you need to book in I class. And (to cut a long story short) what I'm seeing suggests that the I class fare that you can book now for a mid-October departure (which requires 14 days advance purchase) is not valid for inbound travel after 31 October, thus pushing you up to R class for the inbound half; whereas if you book for outbound travel on 21 October (28 days from today) or later, you can book a 28-day advance purchase fare which allows inbound travel in November as well as October.

So the reason that the dates on which you want to travel are expensive is because of fare rules, not the more common problem of a lack of availability.