My trumpet vines do really well tell they get about 14law" and then the leafs start drying up.
Trumpet vine needs regular watering in hot or dry weather. It also needs well-drained soil. It readily grows in forests which tells me it likes some afternoon shade. If the plant is very dense and top growth totally shades the bottom, leaf drop in the shaded area may occur due to lack of photosynthesis. Is the plant old and in need of rejuvenation?