Feed them a miraclegro-like fertilizer at 1/2 strength. That never hurts. I also highly recommend that you water with 1 tablespoon Epsom salts to a gallon of water for each plant every 4 weeks. That will bring your leaves back to a vibrant green. The combo of fertilizer and Epsom salts will also stimulate blooming.

Yellowing leaves is common for hibiscus and it could be caused by a number of reasons such as not enough sunlight, too much rain water, not enough fertilizer, too much fertilizer, etc.

fortunately, they usually recover quickly even after a majority of the leaves fall off. If your not feeding them, I would try some fertilizer with a N-P-K ratio of 2-1-3. Always make sure the middle number is very low, and the last number is high. Hibiscus can be somewhat picky feeders :)