A little futzing revealed the answer:

When I put all of the child #endregions within the curly brace that teminates the class, alll was well.

>>I have a large class with a lot of methods (I probably will break it up the next time I add to it) and I have used #region foomethods / #endregion pairs to make it more readable.
When I nest more than 4 pairs in a parent pair, the class compiles OK but the - and + marks don't appear on the 5th region and I can't expand or collapse it.
Is that a limit or am I doing something odd again?