When someone blocks you on Twitter, you're no longer able to follow him, and tweets from you that mention him will not show up on his "Mentions" page. The reasons for blocking someone are myriad, and talking to the user directly (on a channel other than Twitter) might be the only way to find out the real reason. Although there's currently no application that will give you a list of everyone who's blocking you, you can check on the block status of individual users by trying to follow them.
Sign in to Twitter and go to the user's Twitter page.
Click the "Follow" button.
Look for the message: "This user has blocked you from following them" at the top of the page. If it appears, the user has blocked you; if not, the "Follow" button should turn green and read "Following." Click the button again to "unfollow" if you didn't actually mean to follow the user.