I don't think there's availability in the I bucket, which is the £1680/£1690 fare for those dates. Well, not on the return anyway, which is, by the look of it, a J fare.

Reckon you need to go about ten days later to get the cheaper fare.

On ITA, make sure you have "check availability" ticked. That'll show you straight off whether the right fare buckets are still available.