Hi,
This probably means that the option "Exclude zero depths" is checked (under Track import options on the GPS import window), and the imported track-points do not have depths. When all track-points with zero depths are removed, there are none left to import into the track.
The issue here is that we have not yet updated RM to use the Lowrance GPX depth extensions, so we are not importing depths from the file. The basic GPX format does not include depth for track-points, and, when available, these are provided as manufacturer specific extensions. We currently support Garmin and Humminbird extensions but Lowrance have only started creating GPX files with depth relatively recently. We will update RM to support Lowrance GPX depths, but at the moment you need to use the original sonar logs if you want to use Lowrance data in RM.
Matt