If you are using Twitter and are trying to follow more than 2,000 people, you may encounter the following message:
"Your account may not be allowed to perform this action. Try logging out and in again."
Do not despair. What you need to do is get more followers. When you reach around 1850 followers, then you will be able to follow more users.
So, you may want to unfollow some people you are following who are not following you back.
I use a tool called TweetAdder to help manage my Twitter followers. It helps me find users who are interested in my Tweets through targeted searches, and it also helps me unfollow users who are not interested in following me back.
And there's that.