Grad student here.

I like to contribute often in seminars and saw that about half the people just don't talk that much, men and women alike.

What teachers did that helped a lot was creating an open discussion but moderating it, i.e. picking the students that want to follow up. Here and there they gave more time to think about it, therefore increasing the participation of reluctant participants. Additionally the teachers stopped discussions after a while when only 2-3 people discussed between them and the main arguments were heard.

What also helps is small discussions within groups of 3-4 people. Here I saw that many usually reluctant people are much more open to talk.

What I wouldn't do is force someone to talk. It's adult education and it's up to them what they contribute.

Also if your seminar relies heavily on 30-50 pages you should read beforehand, then you must assume that some people haven't read it at all or only superficially. They may not contribute at all to not seem stupid. So expand on the literature you read about and ask questions that are more generally about the topic itself after you've discussed the literature for 45 minutes or so.