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The fruit may not be getting enough phosphorus and potassium in the mix. Alternatively, they could be getting too much nitrogen. Here is a link that will help for all general citrus: https://www.gardeningknowhow.com/edible/fruits/citrus/citrus-fruit-thick-rind.htm

The two most common causes for this condition are: Too much nitrogen fertilization, or not enough phosphorus. Often these problems are one in the same, and other times are not the real issue at all. Sometimes it is actually the pH of the soil that is off kilter.

Here is an article that will explain things in further detail: https://www.gardeningknowhow.com/edible/fruits/citrus/citrus-fruit-thick-rind.htm

Being an aged tree such as this, it could be that the tree is nearing the end of production. Although they can produce for around 50 years, this can be more or less. You can try to add fertilizers high in potassium (K) and phosphorus (P), and add a little iron sulphate to help with the pH of the soil. This can help the tree to have what it needs to bear fruit properly.

Here is an article for more information: https://www.gardeningknowhow.com/edible/fruits/grapefruit/grapefruit-tree-not-producing.htm

Too much nitrogen fertilizer can inhibit fruit production and cause small fruit. Or, leaving too many fruit on the tree can cause each fruit to be small. In that case, they should be thinned. Other possibilities are here:
https://www.gardeningknowhow.com/edible/fruits/grapefruit/grapefruit-tree-not-producing.htm

Birds of paradise most commonly turn brown if they have been affected by frost or exposed to a drought. If your area has experienced freezing temperatures lately, this is probably the cause. It's best to wait until spring (when there is no chance of frost and when plants start to grow again) before pruning, which will help the plant avoid any further damage. Then, you can remove the damaged parts or cut the entire plant down if it all seems to be dead. It may still come back from the roots.
https://www.gardeningknowhow.com/ornamental/flowers/bop/bird-paradise-plant-freeze.htm

Most citrus, especially those started from seed can take anywhere for 3 to up to 6 years before you see the first flower. If this grapefruit is from seed, then it is not going to be a grapefruit. The way the pollination works to get these makes the seed revert back to the parent tree.

If your grapefruit was purchased as a mature grafted plant, then it could be something as simple as a lack of fertilizer.

You should fertilize these once a year with a citrus fertilizer.

This article will give you more information:https://www.gardeningknowhow.com/edible/fruits/grapefruit/growing-grapefruit.htm