If the Black Kow was added to heavy clay soil, it's entirely possible that the result holds too much water and too little air. Azaleas and rhodoodendrons need soil with a high level of organic content - 25% to 50% - and a great deal of air space. Often composted manure products are composed of very fine particles which do not allow for much air space at all. They also may not provide nutrients in a form usable by azaleas because the pH is too high.

It's also not clear if the azaleas are evergreen types or deciduous ones.

Echoing maine grower, Black Kow is not a good choice for azaleas. Azaleas need drainage, acid soil, drainage, some shade, drainage, organic matter, and more drainage.

The best fertilizer is HollyTone. Now is the time to apply but only apply at half the rate on the package and only apply once around bloom time. If you see any chlorosis, yellowing between green veins, then more acidity is necessary and powdered sulfur is the best long-term solution for that. It may be worth you while to replant them on raised beds to give them more drainage in your clay soil. That is the normal practice in dealing with poor drainage.

Here is a link that might be useful: Raised Beds

Not that rhodyman's advice isn't excellent, but it's worth stressing again that Rhododendrons and azaleas are essentially pretty low-soil fertility plants, period. They just don't need a lot. I've had some rhodies in the same spot now for 6 years, and every year they grow and bloom like crazy even though they are under a hungry and thirsty Prunus serotina and I've never fertilized the soil and doubt the previous owners did, either.
I'd say if they are newly planted, give them at least a couple years to grow roots in search of nutrients. I even knock as many of those fert. pellets off the top of the root mass as I can. Then, if they are growing VERY slowly or seem to have chlorosis or whatnot, maybe give them a little, like Rhodyman's 1/2 label rate advice. (not sure if Hollytone has Mg, Ca & Fe?) I've only seen about 3-4 plants out of my 60+ though ever seem to need anything other than water during droughts. I know my soil isn't full of nutrients because other plants that are hogs, like Brugsmansias, don't grow at all unless they are fertilized.

HollyTone has only organic nitrogen, many beneficial microbes and the following:

Total Nitrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.0%
0.3%. . . . Ammoniacal Nitrogen
1.2%. . . . Water Soluble Nitrogen
2.5%. . . . Water Insoluble Nitrogen
Available Phosphate (P2O5) . . . . . . . . . . . . . . . . . . . . 3.0%
Soluble Potash (K2O). . . . . . . . . . . . . . . . . . . . . . . . . 4.0%
Calcium (Ca) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.0%
Magnesium (Mg). . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.0%
0.7%. . . . Water Soluble Magnesium
Sulfur (S). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.0%

Derived from: Hydrolyzed Feather Meal, Pasteurized Poultry Manure, Cocoa Meal, Bone Meal, Alfalfa Meal, Greensand, Humates, Sulfate of Potash, Sulfate of Potash Magnesia and Elemental Sulfur.

*Contains 2.5% Slow Release Nitrogen.

It does seem to be missing iron. But a companion product IronTone does have iron. It is a 2-1-3 fertilizer with 5% iron and 13% sulfur. You could alsoI add iron sulfate, especially if chlorosis if a problem.

I agree that most rhododendron and azaleas don't need much feeding unless you are having a problem. My soil need phosphorus and sulfur once in a while, but that is all.