My trumpet vine hasn't bloomed in the last 2 years. It used to be covered in flowers, but now it only has an occasional few.
I would guess that the nitrogen to phosphorus ratio is off. The soil may have an abundance of nitrogen, which would result in nice, healthy green growth but little to no blooms. Without the right amount of phosphorus, it will not bloom well. Bone meal is a great way to add phosphorus. You may be able to speed up flowering by pruning back the leaders to encourage more side shoots, where the flowers appear.