Now ain't this weird...
I went back to the same sketch after closing and re-opening, and was able to select, dimension, extend whatever... BUT... this was when the sketch was in Isometric (rotated) view. When I selected the "Look At" Icon, and selected a line to "Look At", then bingo... same problem-unable to select, extend etc.
Why would selection NOT work only in the "Look At" View.
Now I am real puzzzzzzled.
Regards,
Don A 🙂