Do you mean that the flowers fall off but no cherries form?
What sort of cherry have you got?
You can't stop them falling. If they are fertilized they will stay on the plant and grow to be cherries.
Do you know what kind of cherry tree it is? Importantly can it self fertilise to produce the cherries or does it need a partner nearby (which would potentially explain a lack of cherries)?
Or are you getting cherries develop and then fall off? Do you have birds /squirrels that might be pulling them off to eat?