How to get people respond in my older discussion?

The older discussions are still there, it's just that they are at the bottom of the pile. You can go into "discussions I've started" and scroll through until you come to the one that you want to "revive". Just post a comment to one of the posts and it should then move up into recent discussions again. Hopefully somebody else will see it there and respond again to keep it going. If it is a great topic but you feel that it did not get enough responses, you may even try to start a new discussion by rewording and changing the initial discussion.

Simple way - make more friends and ask them to repond to your old discussions. Dont forget to rate them as well!