Asked April 23, 2021, 10:57 AM EDT
Montgomery County Maryland
The amount of direct sun this bed is exposed to might be a bit high for azalea preferences (during summer, a least); it's hard to tell since western sun is stronger and the mid- to late afternoon timeframe suggests the light persists for possibly 5-6 or more hours worth. Azaleas being woodland-adapted can struggle in sites that receive too much strong sunlight, especially backed with a heat- and light-reflective/radiative surface like a brick wall. While this doesn't necessarily explain the ailment of the struggling lone plant, it is a factor to consider that can contribute to the vulnerability of the plants to other issues. If the sun isn't that intense here, then the exposure should be fine.
It's possible the root system on that single struggling plant wasn't in as good a condition as the rest when it was planted, and the transplant shock took more of a toll on its condition. Flowering on spring bloomers like azaleas primarily takes place as merely the absorption of water to expand the buds, since bud development itself occurred last summer/fall. Thus, if buds or subsequent new leaf growth fails to open, that suggests the root system isn't functioning properly or has insufficient moisture (usually the former, especially since the latter is easy to monitor). Azaleas are very sensitive to over-watering and poor drainage; while you mention the low likelihood of that being an issue here overall, drainage can be surprisingly varied even within a bed of this size. It's also possible its root health was compromised by water issues before it was purchased, as such circumstances can take time to reveal symptoms, especially with evergreens.
In the future, it's best to not use pure amendment in a planting hole, but rather to mix it in thoroughly with the existing soil. Use minimal amounts of amendment, as the more of a difference in soil types there is between the planting hole and the surrounding soil, the greater the likelihood the roots will respond negatively when eventually encountering native soil; plus, this creates drainage problems as water doesn't move through the two soil textures in the same way. Approximately a couple of shovel-fulls of amendment (topsoil, compost, and/or fine-grade pine bark mulch) should be sufficient to amend the soil for a single shrub of this size.
Similarly, make sure root balls are well-loosened upon planting, as those bound from their time cramped in the pot will struggle to reach-out into the surrounding soil to establish themselves. Azalea roots grow relatively shallowly - often not even filling the pot to depth - and so become fairly easily entangled and matted when reaching the edge of the container.
Holly-tone is a suitable fertilizer for acid-loving plants, but should not be needed for either freshly-planted or even established shrubs unless signs of nutrient deficiency appear. Too strong a nutrient level can actually damage roots, though fortunately organic fertilizers like this break down more slowly than some synthetic formulations. You don't need to worry about the fertilizer you already applied, but we don't suggest using more at this time.
For now, just monitor the plant's status, check for watering as needed, and see if it puts out new leaf growth. (Not flowering isn't too troubling a sign by itself if all else looks good.) If you don't see signs of active growth by May or June, the plant may be too compromised to recover. If the existing older foliage appears to stay healthy throughout the spring and summer, then perhaps this year's growth buds were damaged due to cold or water stress and the plant will have to wait until autumn to grow more for next spring.