Petra, your situation reminds me a bit of forums like software/hardware help forums. People show up, ask a question, get an answer from an expert, and leave...
I think you'll have to do a couple of things to build a community. First, be sure you have a topic or two that are designed to encourage discussion rather than just Q&A. You might even consider an Off Topic section, but you'll need to get your community going before people would consider posting OT stuff. Devote some thought to what topics might get people talking. If the topic of the forum is immigration, how about "Immigration & Family Issues"? I hesitate to suggest this as it is likely to be a moderation burden, but how about "Immigration Politics"? You know the kind of people that post - figure out what they would want to talk about or share with each other.
The second thing is to be sure you've got some discussion facilitators. Many forums develop these on their own - members who are friendly and get involved in almost every thread (when there are few threads). If you don't have anyone like that, you may have to fill that role yourself. Start some discussions with provocative posts - "Should Quotas Be Cut or Increased?", "Should the Government Crack Down on Illegal Aliens?" - and keep the conversations going until they catch on their own.
I recently paid a visit to my local IRS office. People took a number, got called up, asked their question, got an answer, and left. Nobody hung around to chat, needless to say. That's your forum now. Think of how you can make your forum less like that sterile office and more like a neighborhood pub. The guy at the end of the bar answers questions, but you are the bartender who welcomes newcomers, makes small talk, introduces people, and keeps scanning the room to be sure everyone is taken care of. You'll be successful when the pub is filled with the hum of conversations at multiple tables and people hang around for hours, night after night. :)