What file format are you using?

I did a quick test by Exporting an Invoice to Excel 2.1 format and then importing it back in. The first line of the Text lines was about 50 characters and it imported it properly.

However, if I go back into Excel and drag the column separator to narrow the column, then the only part that will be imported is the visible part of the cell. If you are using Excel, make certain that the columns show the full width of the text.