Good line of sight to the Marine Traffic receiver site near you? Are you sure it's a Marine Traffic site? Have you tried aprs.fi? They use different receivers. Do you know for sure you are transmitting?
Where in MD are you?
Marine Traffic did pick me up once while going under the Chesapeake Bay Bridge, so I think I'm transmitting. My slip is in Chesapeake Harbour near Annapolis. Perhaps the base station listed on the proAIS2 target list was not a Marine Traffic base station, but aren't there any others near Annapolis? At what sort of range should I be visible with my Raymarine AIS650? I would think I would be showing up much more often.