My plumeria is growing new leaves, but lower ones are turning yellow, drooping.

It would help to know where you are on the planet. If you are in the northern hemisphere, 'tis the season for them to go dormant.

dagosta. I agree with Carol.

Welcome, @dogosta. You do need to put your location in your profile so that it shows when you post.

Like last fall, I have put some plumeria in the solarium, at least those in bloom, with flower stalk, and/or with seed-pods. Even though they get lots of natural light, water, and occasionally, diluted fertilizer, every couple of days I will remove a yellowing leaf or two. I can hold off them going dormant, at least to a point, but slowly but surely, they'll all go dormant and lose most of their leaves and quit blooming. Though the low temperature never gets below 68 F (that's what the thermostat is set at) it also never gets hot because the space is climate controlled.