Why do blueberries fall off before they are ripe?
The first thing that occurs to me is lack of pollination. Do you see any bees on the flowers during bloom. Are all your bushes the same variety? Blueberries benefit from cross pollination.
As Mark suggested, pollination may be an issue. Another possibility may be a disease called mummy berry, whereby berries turn pink or tan, become ridged, and fall off the bush before harvest. In that case, the berries are somewhat firm and have a blackish structure inside. Yet another possibility is fruit worms which may web berries together and cause them to fall off although you would see holes in the berries and frass (insect poop). Lastly, blueberry maggots or fruit flies may cause berries to soften and fall off before harvest. If you have more observations regarding when it happens, that may be helpful. Best wishes,