I have had ny Trumpet vine for almost 10 years. Ome year I got 6 blooms. This year I had several blooms but they shrivel and die when they are VERY small and then the end of the vine drops off. The foliage is lush and lovely. I cut it back each fall. It is on a south-facing fence in full sun. I have been giving it Jack's Blossom Booster 10-30-20 for two years.
It could be a pollination problem. When the flowers are not pollinated, the plant "aborts" the bloom. Or, most likely it is petal/flower blight, a common fungal infection, which causes the blooms to become brown and shrivel. The petals may even clump together but not drop off. Removal of all affected petals, including those on the ground, is recommended. Applying a specially formulated fungicide can sometimes alleviate the problem. In severe cases, pruning may be needed.