Just bought a home with a mature grapefruit tree, super juicy sweet pink grapefruit. Don't know what to do to keep producing the best.
Lucky you! Pick the grapefruits if they are ripe. The longer they stay on the tree, the sweeter they get, but you don't want them overripe and lumpy.
These articles should help:
Why do my grapefruit fall off the tree when they're still green?
Some possibilities: moisture problems (too much or too little), nutrient deficiencies (typically potassium), a pest infestation, sudden changes in temperature (when temperatures spike or fall) and a small degree of self thinning can be common in June, etc.