For those scenes, the neg may well have been under-exposed, or been allowed to gain too much contrast in development. Flashing will make it easier for you to get a tone in the area of the sky, but if the sky is plain on the negs then of course there won't be any detail to see in the sky areas. It could well also help in detail of the smaller branches against the sky and with any minor flare effects there.

A suggestion for a test strip for the flashing - cover a part of the long side of the strip, then you will have a reference of paper-white to compare to each exposure step.