When Smartling imports existing translations, Smartling treats them as "unmapped" strings because the segmentation and field structure of the imported translation may not match that of the original string.
If a string has multiple fields (snippets) due to its HTML structure, Smartling cannot reliably associate the fields of the translation with the fields of the original. The Translation Interface will display a warning when the HTML structure of the Translation doesn't match the source.
Resetting the translation fields forces the translation to assume the exact field structure of the original. Smartling places the text of the translation in the first field, and you can then cut and paste the text into the appropriate fields, which are now properly mapped to the fields of the original string.
Click Reset in the warning message or Ctl + Alt + R to reset the translation fields.