This is possibly a problem with chart categories on your unit. Some firmware have an issue whereby categories are not updated properly when maps are changed, so the first thing I would suggest is a soft-reset (hold "pages" button whilst starting). See if that makes a difference. Also check which chart categories you have available in your unit once the map has loaded - you should see bottom composition in there, and be able to turn it on or off. Also, of course, make sure that your bottom composition isobaths are visible and showing within ReefMaster before making the export.