I could not find specifics on Excel import, but might suggest that ONLY .xls can be imported and not .xslx Since the Save Records as Excel only list .xls, we could be on to something.
Can you post screen shot(s) of your import dialog and show the 'types' drop down?
FM 9 User Guide is here:
we encountered the same in the past using fmp (only xls possible).
it's so easy to convert xlsx to xls that i shouldn't bother about the WHY and HOW.