One of the most common questions I see in garden magazines is also the one I hear the most from gardeners: "I have had an X plant for Z years and it has never bloomed. Why?"
This same question in a regional gardening magazine about a 4 year-old gardenia that had never bloomed had a disappointing answer. They didn't mention what I think is perhaps the most common reason for lack of blooms--excessive nitrogen and not enough phosphorus in the soil.
In the end, they did recommend getting a soil test (which could take weeks or months to get an answer), so that will help possibly help resolve it. And they did remark that sunlight and adequate moisture were necessary.
And I'm assuming (as the writer probably did) that the gardener isn't whacking it back every year in the spring and unwittingly removing flower buds before they open.
But just as nitrogen is the key building block for leaves, phosphorus is the key nutrient for flowers and fruit. And soils in the southeast generally are rather low on phosphorus compared to what a lot of non-native plants like gardenias would like.
Not only that, but even if there is enough phosphorus, a walloping amount of nitrogen will choke off the flow of phosphorus. Meaning too much nitrogen = not enough flowers. So stop throwing high-nitrogen fertilizer all over the place.
Also, too much nitrogen can just make plants look sexy to all the bad bugs and diseases out there. I keep my garden lean on nitrogen for that reason. The time and money that some high-nitrogen gardeners spend on sprays for bugs and diseases (and questions about lack of flowers) I spend in a hammock watching my lush garden sway in the breeze.
So if your shrubs--or perennials, trees, vines, vegetables, etc.--aren't blooming, but they are getting enough sunlight and water, then the likely problem is lack o' phosphorus.
A liberated gardener can remedy this inexpensively and quickly with a five pound bag of bonemeal or rock phosphate. Every year or two I throw about 2-4 fistfuls of bonemeal at each shrub's rootzone, depending on size and whether they've been growing and flowering at a good clip. Organic fertilizers don't much dissolve in water, so they are accessible to the roots slowly, but inevitably.
For perennial beds, every year or two, I throw about 5 lbs. over 100 square feet or so. Every garden is different, so if plants are madly growing, forgo fertilizing, as it will go to waste and perhaps wash down into a creek, which would be bad for the fish.
How to choose between bonemeal and rock phosphate?:
Bonemeal is made from bones from slaughterhouses--otherwise it would end up in a landfill. Some gardeners like to repeat an old canard that bonemeal has no nutrients because it's been steamed to kill microbes, but that just isn't true. Bonemeal generally has about 10-15% phoshorus, about 3% nitrogen and little or no potassium and a full range of micronutrients. It also has a good amount of calcium, but not enough to really effect the acidity of your soil. If using bonemeal gives you the willies or bothers your conscience, use rock phosphate. If you have dogs running around in your garden, you may also want to use rock phosphate. When your dog gets a whiff of the bonemeal in the soil, they may start thinking that there's a cow in there somewhere and they'll find it if they can just dig deep enough.
Rock Phosphate is quarried from open pits and consists of crushed, compacted fossils of sea creatures, so it is high in phosphorus and calcium from their shells and bones. It won't attract dogs or torture your conscience as much. Only has about 3-5% phosphorus, but also has other micronutrients.
So after you've dosed your garden with some flower-forming-phosphorus be sure to fix yourself a tasty, nutrient-rich potion too. I'm thinking a cold Bloody Mary with a crisp pickled okra. And a garden magazine.