Confederate jasmine or star jasmine (Trachelospermum jasminoides) is not a true jasmine flower, despite its name. However, it produces clusters of white, highly fragrant flowers in the spring and summer. If these flowers are turning brown, it is most likely due to improper care.
Star jasmine vines prefer to grow in full sunlight to partial shade. If grown in full shade, it is unlikely to produce flowers. If something obstructs the plant's light, its flowers will turn brown.
Star jasmine plants like to grow in moist but well-drained soil. Too much water or too little water can cause the flowers to turn brown and shrivel up.
If your star jasmine's leaves turn yellow, it is probably because the plant isn't getting the proper nutrients and that you should add fertiliser. For best results, fertilise your star jasmine plants in the spring.