For 6 mos. I've had beautifully flowering hibiscus plants in lg pots outdoors (5-6 hrs sun) and occasionally spray for whitefly and aphids when suspected. Lately ALOT of leaves are yellowing on each plant and buds are developing but never flowering, then drying up. It's been pretty warm here in SoCal lately and I've been watering every couple days. What could be happening? (I have alot of my time & money invested in these plants, and need help.)

Kinda hard to say what the problem is without actually seeing the leaves. The plants do prefer full sun, and moist but not soggy soil so it sounds like you have that covered.

The plants are prone to powdery mildew, scale insects, whiteflies and mealybugs so be sure to inspect well for any of the above and treat accordingly if found.

My first guess is a nutrient deficency. Make sure you are fertilizing with a half strength liquid fertilizer every time you water.