Update: I've also tried .csv, .txt (tab delimited), and they're working for all tables except one. This table doesn't seem to have anything different from the others, but for some reason it just won't import properly.
Either the fields don't match up (all the fields from the xlsx automatically correspond to the first field of my Filemaker table, which isn't right) or the fields match up but the short text fields end up blank on my filemaker table.
If you export to XLSX or XLS files, looking at the problem fields may give a clue.
I would be tempted to open the XLS in Filemaker, you should get clean data and then import into your designed database.
There is a utility to do many conversions - www.fmpromigrator.com/