I got it to work.
I noticed a small white vertical line in the middle of the screen
every time I gave the combine command. It was about one
centimeter tall. I pulled on it with the cursor and it opened
into a baby window. I clicked on the command icon and the
entire combine window opened ..... weird.
I quit Acrobat and reopened it and then gave the combine
command again and it opened correctly.
Now it works.
The combine window apparently had collapsed into a small
vertical line. ..... seems very strange.