Sorry for the slow reply...busy grading research papers etc! Anyway, I figured out my issue. MY MISTAKE!
I think I accidentally deleted some important columns in the spreadsheet, including the "File key" column. I often use the "Hide" function to hide these from view, so I can see all of the rubric scores on my screen, as well as additional columns I have created with functions such as averaging, subtracting points for days late, etc.
The "Hide columns" command is not visible from the regular menu, but only from the context menu when columns are selected. The "Delete columns command is only two commands up the list. I must have been tired or working too fast, and clicked the wrong command.
I went into the Version history and restored the version before I made that error. Luckily, Goobric had saved scores and comments in documents I had graded since then, so I just clicked "Submit" again, and the scores populated in the spreadsheet.
Thank you Goobric! Great product!