I have a grapefruit that bears prolifically but the fruit does not mature
Certified GKH Gardening Expert
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.