I have a grapefruit that bears prolifically but the fruit does not mature
This can take nine months or longer sometimes. If it is well passed that, and still not ripening, then I would suspect that there is, either, too much nitrogen in the soil, or not enough phosphorus and potassium.
If this is not the case, then I would chalk it up to a pH imbalance in the soil. You can mitigate this by adding dolomitic lime, and iron sulfate to the soil along with your regular fertilizer in the spring.