Q.I have 5 gardenia bushes that line the length of my back deck.
All bought at same time, all same type, size, etc.
All get equal amounts of sunshine & water.
All equally spaced between the next one.
So why are 4 of the 5 growing & blooming wonderfully & the one on the end is the runt of the litter?
It doesn’t bloom nearly as much as the others & the shrub/bush/leafy part of it is not nearly as full & filled out as the others.

Certified GKH Gardening Expert
What a mystery. The first thing I'd do is check the smaller plant for bugs, mold, leaf issues or other problems. I'm assuming they're all being fed the same way. Could this one be exposed to something the others aren't? Is it getting a little less sunlight - a little more shade during the day? Perhaps it has a root problem? It may be just the nature of that particular plant, but I'd examine it, regardless. Here's an article that may help:
https://www.gardeningknowhow.com/ornamental/flowers/gardenia/no-flowers-on-gardenia.htm
Our website has quite a few articles about gardenias, but I think this one might be most helpful.