I'm growing some red heritage raspberries. I'm noticing on my plants, each fruit has only 1 drupelete. Nearby, I've got some black cumberland raspberries that have full fruits.
Anyone seen this before?
It is usually due to incomplete pollination.
Berries on the same cluster of the same plant can have a wide variation of filled druplets. Erratic weather combined with bloom time variations are the main cause. Early blooms might fill out, next week's may not, and the week after may revert to filled berries.