I think you are onto something with the idea that is getting too much sun.
The other gardenia across from it that you say you like because it flowers more doesn't have the same sun exposure as the one you put the umbrella over.
I think it is getting too much sun. Some of the other plants you have up against the home on that side look as if they too might be getting too much also.
The umbrella is probably a quick fix for now, but if you really think it is unhappy, mabey you should try it inside, it probably wouldn't hurt it.
How old is the potting soil you have it in?
The soil must be changed at least every 2 years.
So, if you do want to pot it up, mabey some fresh soil will make all the difference.
You have been doing a good job with them though, they look pretty good, even the bush you say looses it buds.