A lack of uniformity in a mundane expanse such as the sky likely traces back to the developing technique. The first thing I would do is re-fix these negatives in the hope that they were under-fixed. The rule of thumb is to immerse a small piece of undeveloped film in the fixer. This is performed in a lighted room. Time how long it takes for the fixer to clear the film, then double this for the appropriate fix time. Better too long than to short.

That being said, development is dependent on the developer, which is mainly water, infusing into the emulsion, so it can go to work. The emulsion binder is gelatin, a long chain polymer. It is chosen because it is transparent, flexible, and has low solubility. When first wet by the waters of the developer, it swells. This action opens up the structure allowing the developer goodies to enter and do their work.

If uniformity is the problem, likely the developing time is too short. My first advice is to select a developer formula that takes more time to work. How about ten minutes obtained by formula, dilution or temperature?

Next let’s talk about “bromine drag”. As the developer works, it reduces the silver salt crystal to its two component parts. The silver portion, now metallic silver, remains embedded in the gelatin; the halogen portion is dissolved and is flushed out by agitation. As the halogen exits, its presence retards development until completely flushed out because fresh developer can’t get in. If the film is in a stagnant fluid, halogens, heavier than the surrounds, flow downward by gravity. The dots on the edge printing will divulge this by showing a faint comma-like tail. Look for this as a sign of insufficient agitation.

Again, my advice is a longer developing time.