Taking a suggestion mentioned in a few comments above, I went thru the entire procedure again with a different (reject) image (i.e., blemish removal & hot spot removal, each on separate layers). This time, however, I kept the retouching on each layer simple; i.e., retouched one single blemish on the Blemish (blank) Layer; removed one single hot spot on the Hot Spot Layer, AND was careful to deselect the patch selection. Happily surprised to see that problem disappeared!!
Followed the same procedure again, but this time, I purposely did not deselect. Problem reoccurred. No matter what layer I activated, Healing Tool simply did not function. Seems to confirm that "failure to deselect" was the cause of the problem. Since I hid each patch selection (Cntrl + H) when I adjusted "the fade", I didn't have a visual selection to remind me to deselect.
It seems, therefore, that I must have one or more active selections still on the Hot Spot Layer that have disabled the Healing Brush Tool (at least, outside the active selection area).
Last question on this tread: Would like to preserve the work I already invested on the Hot Spot Layer. Since the active selection(s) have been hidden in various areas within the Hot Spot Layer (i.e., Cntrl + H), is there a quick way to deselect one or more of these (invisible) selections that may still be active on the Hot Spot Layer? - in one fell swoop.
Hi
A quick way to deselect is Ctrl+D (Cmd+D) on a Mac) or go to Select > Deselect.
When you make a new selection, it replaces the old one. If you modify a selection by adding to it or subtracting from it, it is considered to be one selection.
When you have an active selection, you can only work inside that selection (as you figured out). You are doing well!
~ Jane