I have several plants that are putting out many yellow leaves and the green leaves and blossoms are very tight and knotted with what looks like white foam on them . I don't want to loose them but they are not looking good. I can't upload any pictures at this time.
Potted Hibiscus Plants
Certified GKH Gardening Expert
This could be Mealybug on your plants.
Treat the plants with Neem Oil.
Here are some links with more information.