Can`t give an answer...but mine have done the same.

Frost damage to blossom, very hot weather as fruit is forming, poor pollination, and damage by mites. Fun growing cane fruit, isn't it?!
All you can do is provide the best growing conditions so that damage is limited. Generally cane fruit is tough stuff, but I've certainly noticed a poor crop of blackberries first off this year in some places. Having said that, I've also seen some beauties, so it's a patchy year. Growing autumn fruiting raspberries to extend the picking season and avoid some of the earlier pests is one way of ensuring you get some good berries. Loganberries like a warm sheltered site, good soil, and then you pray for bumblebees which tests have shown are best for cane fruit, especially raspberries.