Probably poor pollination of individual berries during bloom. This varies from year-to-year depending on weather and bees.

I haven't looked closely at neighborhood raspberries other than to see some are already ripe, but usually a percentage of raspberries remain tiny and turn dark.

What percentage of your berries have become full-sized?

larry thanks about 75% are full-sized

That may not be bad considering the poor spring to date. I haven't studied my black raspberries closely, but I did pinch off a couple of unformed berries the other day.