Hi there. Sadly, I now live far away from any marine traffic, and haven't monitored AIS in some time, but I do have a few suggestions. You may have already tried this...
First, have you checked this site to see if there are any vessels within range?
MarineTraffic Live Ships Map. Discover information and vessel positions for vessels around the world. Search the MarineTraffic ships database of more than 550000 active and decommissioned vessels. Search for popular ships globally. Find locations of ports and ships using the near Real Time ships...
www.marinetraffic.com
Secondly, are you using the attached antenna or an external antenna? Depending on where you're monitoring, an external antenna may be needed to get good coverage in the 161/162 MHz range. Or maybe just try monitoring outside--but I'm guessing you've probably already tried that.
That's all I've got... Good luck!