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Soil should actually be watered well, then allowed to dry slightly.
Make the adjustments in care as indicated in the link below.

https://www.gardeningknowhow.com/ornamental/flowers/plumeria/growing-plumeria.htm

Your plant should be mature enough to bloom. Correct fertilizer and application timing is important.

This article will help you.

https://www.gardeningknowhow.com/ornamental/flowers/plumeria/growing-plumeria-cuttings.htm

Yellow leaves could be watering issues. Make sure your watering the plants evenly, moist soil.
Plumerias are susceptible to a fungal disease called plumeria rust. It first appears as many tiny, yellow, powdery spots on the undersides of the plant's leaves, giving them a mottled, yellowish appearance. Eventually, the leaves turn a uniform, bright yellow, then become brown and drop from the plant. If the infection becomes widespread, a plumeria can lose most or all of its leaves, causing severe stress and potentially killing the plant. Once present, it's not possible to cure plumeria rust, but you can help prevent it by planting where air circulation is good and by keeping the area clean.
A plumeria that's infested with pests might develop yellow leaves -- the most likely culprit is a tiny pest called the sixspotted spider mite, although other types of mites can also infest plumeria. Pests can be treated with Neem Oil.

https://www.gardeningknowhow.com/ornamental/flowers/plumeria/growing-plumeria.htm
https://www.gardeningknowhow.com/ornamental/flowers/plumeria/how-to-fertilize-plumeria.htm

These plants needed well draining soil.
Check the soil to make sure it is not soggy. Pull back any mulch to aid in the soil drying if it is too wet.

https://www.gardeningknowhow.com/ornamental/flowers/plumeria/plumeria-flowers-dropping.htm
https://www.gardeningknowhow.com/ornamental/flowers/plumeria/growing-plumeria.htm

It sounds as though you have plumeria rust. Here is a resource you will find helpful:

https://www.ctahr.hawaii.edu/oc/freepubs/pdf/PD-61.pdf

This article will help you.

https://www.gardeningknowhow.com/ornamental/flowers/plumeria/plumeria-pruning-info.htm