Are you acidifying your water? What is the pH of your tap water? Is it highly alkaline? How are you fertilizing them?

Mine starts out at 9.8-10 (certain death for a gardenia). I acidify it down to ~5.5 with vinegar in the hopes that it ends up around 6 after dissolved co2 escapes. The gardenias I have in the 5-1-1 are flourishing. My 3rd set of buds for the season is about to open.

While it's good to have your media pH right where it's supposed to be (somewhere around 5.5 - 6.0), a little lower or higher for a few, it isn't as important as it is in (mineral) soils that depend largely on mineralization for the nutrient supply. If you're supplying the nutrients in a soluble fertilizer, and the pH range is reasonable, the plant will get the nutrients.

Where pH does come to play is where particular plants have difficulty limiting their uptake of certain nutrients or elements. We see this in plants that are particularly susceptible to issues with fluoride, so we need to keep pH above a certain threshold. Another example is Pelargoniams, which have issues with their Fe:Mn relationship that requires monitoring of both the media and irrigation water pH for production.

I think you probably need to supply more info before we can make an informed decision about what the issue might be. Fertilizer type? How much lime? Watering habits (first suspect)? Light levels/temperature? ......

Al

Here's one of my gardenias in the 5-1-1:

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A shot of the soil made exactly as Al suggests including the dolomitic lime. The oversized bark was added to the top for aesthetic reasons:
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I have a gardenia in the 511 mix going into the 4th month and it is thriving and branching out. My only problem is that it only had 1 bloom all season and has not set any buds after that one flower. I don't think it's the mix but some nutrient deficiency.

I have a gardenia jasminoides radicans in 5-1-1, as well, and it seems to be quite happy.

Josh

I guess you have convinced me now that 5-1-1 does work for 'gardenia's. Somehow the plant I had in 5-1-1 had root rot and hit the dust. I just bought another new variety of hardy gardenia named 'gardenia -pin wheel' which is hardy to zone 6. I planted it today in 5-1-1 mix, crossing my fingers that it will survive and do good. I have never checked the ph factor for this mix as all my other tropicals (jasmines, hibiscus) are doing very well in this mix. I do add weak solution of foliar pro with each watering. Let me know if I have missed anything else.
Kandhi