Whole flower stem drops off.
The past 2 years I have a plumeria plant that has dropped a flower stem. Both years it grew the flower stem to about 3 inches with lots of emerging buds before falling off. This plant receives the same care as my other 4 plumeria plants which appear to be growing fine, in fact, a white one produced a flowering stem late last year which stayed on the plant over winter and is flowering now. It also has produced another flower stem on another cane this year that is over 5 inches tall. The problem plant is supposed to be a red and the leaves have a reddish tinge to the underside even when fully mature. Last years cane started to grow 3 new canes after the stem drop and this year's appears to be growing 2 canes. I live in San Jose, Ca. and grow them outdoors from early spring under full sun from about 9 AM until 2 PM with partial shade until late afternoon. Late autumn I move them into a small greenhouse with some Cattleya orchids that has a small temperature control heater set to turn on at 45 degrees. Depending on weather, they get watered every 3 to 7 days and fertilized with 15-35-15 every other week. Thank you in advance, Jim Kawasaki
That is an awfully large amount of phosphorus to be feeding this. It can raise the pH of the soil if you do not add something to bring it down. Iron sulfate can do that pretty well. Also, many of these cannot tolerate temperatures below 50, and this could hinder flowering quite a bit as well if it does get down to 45 at night.
This article will help you with the care of these: https://www.gardeningknowhow.com/ornamental/foliage/yucca/propagation-of-yucca-plant.htm