There are a number of cultivars which have thin branches as one of their characteristics. So it may simply be the cultivar.

I know nothing of Plumeria culture but I notice that your plants are indoors and reaching for the light. That will cause etiolation. All the leaves are angled towards the window. I think they need more light. Can you put them outside for the summer?

Unfortunately no. I live in an apartment building with no place to put them outside. I just need them to make it 1 or 2 more years before I get a house.

I have one that's about 3 yrs old I started my friends cutting and Its doing the same thing and its outside.... I live in Florida. Of course it's the one on the left.

Does anyone know how I can make it sprout lower limbs??

They're just going to be spindly when you grow them indoors. No way around it.