Hibiscus enjoy water and as CA said a balanced fertilizer. I am assuming with your mulching you slowed water intake? Ensure they get feed and watered and I would expect blooms to increase significantly.
I have two on my property and changed them over to drip irrigation/micro irrigation....the bloom almost non stop down here minus when I prune.
10% of your customers are 90% of the problem.
You may think the grass is GREENER on the other side, but if you take the time to water your own grass would be just as GREEN[/COLOR].