Are they in containers and if they are, how big is the pot (diameter across the top) and how big/tall are the plants? If not in containers, have you used bark mulch around them?

They are in a container and under a lean-to area with clear corrugated plastic roof, diameter of pot is about 15 in and pot is about a foot tall. They must be about 7 or 8 feet tall as they have started to creep along the roof of the lean-to, we also have a honeysuckle in the same pot. We are trying to train them across the roof, we have trellis coming up from the pot and wires across the top. Am attaching a photo. The photos are on my profile pictures, I didn't know how to attach them to this message. Thanks for your help.

Hmmm, just had a look at your pictures. I suggest its too small a container for both. The lonicera is very greedy and so the Thunbergia is aborting the older leaves in response to this physiological stress exerted by the other plant(s). The root ball wil be solid with roots I would think at that size of plant in relation to the pot size. Unfortunately, no amount of feeding will really help I would think.