Plumeria Problem

This may be compromised vascular system due to what seems to be past damage to the trunk. If the trunk is soft it may still be infected, with what might have been a bacterial problem. If that is the case we suggest you take a stem cutting at the top and re-propagate the plant. If the stem is hard then you might un-pot it and look at the roots. If they are good then and are not causing the problem then the vascular system in the stem may have been damaged in the past so that the plant can not get sufficient nutrients. Sometimes the plant can grow out of this and perhaps planting in the ground will help or re-potting in new soil. If neither of those work and it continue to not perform then we would suggest re-propagation.