formula working in some cells but not in other similar cells

mcetera 

Thank you to all who have read my post and to those who were thinking of replying.  I have solved my issue.

Good. And how did you know that I (among others, I'm sure) was indeed thinking of replying?!

Frankly, though, my reply would have been along these lines: you should seriously consider updating to a current version of Excel. The dynamic array functions that are now available make handling a situation like yours both easier AND more readily understandable. Less of that "How did that work again? I know I understood it when I first wrote it, but...."

So it's good news that you resolved it on your own. That's always (in my experience) more satisfying, and a better learning experience, albeit frustrating as you go through it.

Have a good year!