Sometimes, just to prove us wrong I think, plants just don't like being in an 'ideal' place. Any chance you could take a cutting and try it somewhere else in the garden.
Some time ago I obtained a double hibiscus which I had been looking for some time. Planted it in the 'ideal' situation and gave it loads of tender loving care. It's response was to look weedy, poor if any growth, no flowers to speak of and leaves that fell off at the drop of a hat, all this over a period of about five years.
As I've only got a small garden I have no room for failures, so moved it to a place which was the opposite off its ideal situation and was resigned to digging it up when it finally died. Well, that was five years ago and it's now 6ft tall 4ft diameter and covered in blooms and people are always pestering me for cuttings. Perverse or what?