The problem is a water problem.
Go outside and notice what is growing behind the gardenias with the problem.
Bright green grass.
The fence is also open behind them.
By the look of your yard, the high spot is suppose to be where you planted the gardenias.
But it looks like they are getting alot of water running from your neighbors yard when it rains, the grass is a sign.
The gardenias in that case, aren't planted high enough, they are suffacating because the water is too much for them and it isn't draining well.
Gardenias hate mulch. They love pine needles also.
Just for the gardenias that don't seem to be doing well, I would add more soil to elevate them a little higher than they are, and see if you can find pine needles, although this time of year you may not be able to.
Remove the mulch, and use pine needles instead.
What I'm trying to say is, they are drowning.
Raise them a little higher on your mound and the next time it rains or you water them real good, go out and notice the drainage situation when its raining.
They will be fine.
Are they August Beauties?
They are resiliant and gorgeous.
Good Luck to you!

I can only second what butterfly said: gardenias need consistent moisture, but too much, especially when they're first planted,and they rot real quick. I'd say that a light mulch shouldn't hurt, but keep it at least 4-6" from the stems, otherwise you're asking for fungus and rot at the plant's crown.

Hope the rest survive and bloom for you!

Jim