I have given both good fertilizer and Epsom salts to our indoor plumeria. We have had the tree for several years and can’t get it to bloom!
Certified GKH Gardening Expert
This can happen when the fertilizer has too much nitrogen. Try getting a 10-30-10 to get it too bloom. They will also want it to be warm and humid to help this. Here is an article for more information on their care: https://www.gardeningknowhow.com/ornamental/flowers/plumeria/growing-plumeria.htm