I solved my own question so I'm replying to my own post.  The solution to Form 8990 not filling in is that I CHECKED THE WRONG BOX in answering the question on Line 10 of Schedule B to the Form 1120S.  I should have checked YES, but I checked NO by mistake.  Answering yes to that question, namely that yes, my corp has sales in excess of $26 million, does properly make Form 8990 appear and fill in the calculated fields.  I had checked no, with the result that Form 8990 would not calculate nor print.

So I'm embarrassed.  And an idiot.  Thank you.