| Suggest you carefully dig up the unhappy gardenia and look at its roots. If it was originally planted in a peaty mix that may be causing problems. One way is that, despite watering, the original rootball has dried out and is not being rewetted. (Peat is quite difficult to rewet...) The other way is that it is holding too much water and the roots are rotting away. Then what? If it is dry put the plant's rootball into enough water in a bucket to fully cover the ball. If it bounces to the surface and floats - it was definitely dry. Leave it in the water to soak. Meantime - head back to the hole you made. Clear away the stones and, if it was plastic weed mesh around it - make a bigger hole in the plastic. (It's really not suitable in areas that are actively gardened for a whole lot of reasons.) Dig over the soil and do what's known as 'a five dollar hole for a fifty cent plant'. If the rootball is four inches across (10cm) dig a patch around 12-15 inches wide (30-50cm) and double the depth of the current root ball.
Add any mature or commercial general compost you have to the soil you've loosened. Make your new planting hole. Then get a bucket of water and pour it in. For preference, the water should drain right away, leaving the ground damp, within say ten minutes. If it's still there after thirty minutes - plant something that tolerates swamps. Go back to your soaking plant. Hopefully the rootball is now damp and you can swizzle it gently to take off the old mix leaving the roots mostly free. Scamper back to your new hole. Use some of the compost-soil mix to make a mound at the bottom of the hole and spread the roots over it. Draped. Fill in the hole. Using a gentle 'rain' setting on your hose or watering can, water in the new plant. DO NOT step around it. Roots need air as well as soil and water. If you think it needs a stake - add that before you plant. Too wet. Rotted roots. Might be possible. Take off the soggy mix by swizzling. You're looking for roots with white-ish tips. If there aren't any and there's very little left at the base of the plant - take some cuttings. (Do a google for a video to see how to do this.) If there are - you will have to cut back those lovely branches to bare minimum because the few roots will not be able to keep the top supplied. Ensure there are one to two leaves on each stem. Put the plant into a smaller pot using a damp non-peat mix with a fairly open texture (you can see airspaces easily.) Open up a plastic ziploc bag and put the entire pot+plant inside. Close it up and put it into a warm place with good light but not direct sunlight. Keep your fingers crossed. Allow anywhere from 4-12 weeks for ROOTS to develop. Do not be fooled by the appearance of leaf buds. After about four weeks you can open the bag to check on how it's doing. Or - buy another Gardenia. But do check the hole first - and be sure that any 'weed plastic' is not stopping water getting through. | 
