99% of the time, this is due to not having an image with a transparent background. Check your white underbase chanel. If there is white data all around the image, then the file that was brought into Separation Studio did not have a transparent background. Here are videos on how to remove them-
Removing a white background in Corel: https://www.youtube.com/watch?v=-UyNVJCTOvA
Removing a white background in Photoshop: https://www.youtube.com/watch?v=SV38L4c4JKY
After removing the background and exporting as a TIFF, there will be a dialog box that pops up with a box that MUST be checked for maintaining the transparency. Make sure this is checked.