1. Upload to Google docs and then export as simple xls (I stole this from a useful Moodler in the Using Moodle forum).
2. If possible, right click and save target as . . . to your desktop xlsx files are much more pliant when they're on your desktop.
3. I find that I have to install the free Office Converter Pack twice before it actually will work.
4. Another really useful point. Don't allow spaces, hashmarks, ampersands, or anything that is not an alphanumeric in the file name.
5. Confirm that it really is an xlsx file. Save to desktop, then open in Notepad or Wordpad. Near the beginning or the very end, if you see the dread "xhtml," it is what the extension claims. Conversely, when you get a .doc or .xls that won't open, use this trick to see whether your user simply changed the extension without saving as.