John:
If you are using GL version 9.0D, it may be that the statement has to be "Collapsed". Does column A show the phrase "||** Generated Row **||" in a number of cells? If it does, that means it is in "Expanded" mode. Click on the "Collapse" button near the top of the screen (a black arrow pointing upwards) then try your changes. The most recent version prevents you from making changes to the statement while it is in "Expanded" mode.