Blackberry question

[Medievalist]

We have quite a lot of blackberries in our garden but one area only produces blackberries with a small number of druplets(?). So they look like very small, not fully formed blackberries.

I notice this is sometimes the same in the countryside - lots of normal looking blackberry bushes and then a few bushes with what look like incomplete berries.

Does anyone know why this might be?

I had always assumed small berries were due to poor pollination. Maybe that strain has less appealing nectar?

Lack of light maybe? Sometimes with bushes etc, just give them a heavy prune low down to get sunlight in and air. Also if its quite dense, will stay wet. Wouldn't harm it to give it a feed too.

I mean cut out thick older vines. That will give the younger ones time to grow nicely with nutrient supply for next year.

The one that's most affected was cut back quite heavily last year (it had the same issue then). Gets loads of light and is always full of bees etc.

Not sure what else I can do. The soil here isn't great but other fruit bushes are thriving 🤷‍♀️