I have done so many discuss comments but there is no change in my keyword ranking, please tell me why?
You should try dofollow blog commenting from following site: dofollowblogcommenting.co.cc. They provide the best dofollow blog commenting service.
If you mean Disqus comments, I believe they made some changes so that your comments would no longer have indexed links some time back. I could be wrong, though.
The comments which you are going to do should be valuable and genuine according to the post. Your comment should be look like that you are aware about the matter on which you are commenting
If you are genuine reader then you would definitely comment which is related to that post. The comment you are providing should have clear meaning and make sense. I usually delete Thanking comments on my personal blog which clearly look spammy.