I have never tried to root a plumeria cutting in water. I don't know why your plant doesn't root. Perhaps all the energy is going into the inflo. rather than into rooting. Margaret,
@margaretKevorkian you know that plumeria love heat and sun, and the hotter it is (within reason of course) the better they perform.
I see no reason for you not to use (live) Spanish moss in that way. I did not realize that Spanish moss grew in Fresno. Until my wife moved to Mississippi, she said she had never seen Spanish moss. She spent the first 20 yr. of her life in Fresno.