This issue is caused by the Column properties in Excel not in the correct (Text) format.

 To correct this, save your Excel file as a .CSV file.

 Once saved open a New Excel Workbook. In this Workbook click Data > Get External Data > Import Text File.

• Step 1 in the Text Import Wizard  - Select Delimited
• Step 2 in the Text Import Wizard  - Delimiters need to be set to Tab & Comma
• Step 3 in the Text Import Wizard  - Column data format must be set to Text. Be-sure to select each column an set Column data to Text.

Once the Data is imported Save your new Workbook as and .xls

Now re-map your label file to the New Data source file you've created.