Have you used manage database | fields to:
Change the name of a field in either file?
Added a field to either file?
Deleted a field?
Those are the most common sources of trouble when using Filemaker's Import Records tool. The best way to avoid these problems is to use the Matching Field names option, but this only works if you have identical field names in both files and this is not always possible or practical.
Save the TextWrangler file, open in excel, save as CSV or tab delimited and try a manual import?