That would explain it - those are weekend dates, and weekend tickets are frequently released out of synch with weekday ones - generally a week or two later, sometimes more. The previous weekend (29-30 June) £12 tickets are available on just about every train. If you are patient they will become available for your weekend as well.
My guess is by the end of this week - Thursday or Friday. There's no rush - they won't sell out the moment they go on sale, and the £76 tickets (which I know you don't want) will never sell out anyway