Colleen,
At any time in working with either file, did you turn off workbook sharing? This would clear the history, and cause you to not be able to merge them. Also, have you merged them more than once?
When I teach merging workbooks I create one with generic text and numbers all over the spreadsheet (names, dates, times, currency, etc.)
1. Then I set it up to share, and save it.
2. I then put it in a shared folder on my desktop, make multiple copies (one for each student - with their name as the file name).
3. I then invite them to each open their document, and make changes to it. Anything they want (add & delete data, add and delete cells, change formats, etc.) Save and close to same shared folder. Make sure everyone has CLOSED!
4. Go back to my "original" and open. Merge the workbooks back together (selecting all of the files at the same time).
There are several reasons I do not use "realistic" data in my example. They don't get to see some of the biggest issues with merges that way.
Lets say you have 8 copies to merge together, and only 5 are ready but you want to start working on it. If you go ahead and merge those 5 copies, then later pick up the remaining 3 - if any cells in those remaining 3 where already changed by the previous 5; then you won't even see their changes for those cells, because you already "accepted" a change for that cell.
Formatting changes don't prompt you to accept/decline them, and most people are amazed by this. So, you get lots of colors/fonts etc, that people weren't expecting.
Best Wishes,
AngO