Can you describe precisely what you are doing when you export the files and then precisely what you are doing to import them.
Are you Importing to an existing data base or format? If so have you ensured that the export format matches?
Which version of Access are you using? (for completeness of information).
Whilst exporting to fixed width files and importing to Access should be relatively straightfoward that is only the case when everything is fully compatible and it does not take much create a mismatch!