I know a lot of gardeners have trouble growing gardenias. People try all kinds of advice to get those things to hold their blooms, grow, or even flower. Or even just: not die!
Now, I never tried to grow a gardenia before I moved here so I'm not positive, but I truly truly believe the trouble is that despite what those evil lying plant tags say, they (gardenias) actually have a very limited growing climate. If I had to guess I'd say they like: humid as you can make it, zones 8a-10a, warm nights, part shade, crappy sandy soil that drains faster than a pasta strainer. Thats what they like. And even the part shade isn't that important, but the other things are.
Now why do I believe this? Namely:
Yes, thats right, this is growing in the 'hell strip' outside of my next door neighbors house, beside the cement. There's another one in a similar hell strip situation around the corner, in half day sun living it up too. Nobody waters these. Nobody fertilizes, trims, or cares for them in any way. I suspect the garbage collectors regularly smack em up with the bins. And they look just as good as the two I actually throw some water on every so often in my garden. Around the block a lady has them in patio pots in full sun and those suckers bloom themselves silly.
So here is my advice... if these plants grow literally like weeds in places they like (they compete with the oleanders and crape myrtles for the hell strip spaces) and you are struggling like crazy: it is not meant to be. These plants, I suspect, have a narrower band of adaptability in reality that plant sellers would have you believe. For me, because of these same climatic conditions I can not grow most spring bulbs, I can't grow lilacs (or I can they just won't flower...sound familiar?), I probably can't grow half of the things that are in commercial garden existence. So sure, try them out, but trust me, if they aren't working: its not you, its them. They honestly don't care how nice you are to them, they are extremely picky about things you can't control.
I am coming to some of these same conclusions myself over what is just not meant to be.. it isn't worth wasting my time with in my Charleston climate. Because no matter what zone you are in, there is SOOOO much that will grow!