When you are using the filter criteria, "Imported from (specific file)," you will only be able to filter through your 40 most recent imports. If you are trying to target people from an older import, you can still run a filter by any tags that you used to identify people from the import, or any lists they were added to.

Alternatively, you can filter by "Created at" or "Updated at" if you recall the date range in which your target audience was added to your database.