Make sure your tree is getting adequate sunlight. Fruit trees in need at least six hours of sun and preferably more to fruit well. Fertilize it periodically with that fertilizer you purchased at a rate of one cup per inch of trunk diameter. Water as needed to keep the soil moist but not soggy wet. Citrus trees need good drainage around their roots to do well.
I hope this helps. Good luck with your grapefruit tree!
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