You've probably hit a follow limit. Twitter has imposed reasonable limits to help prevent system strain and limit abuse. If you hit a technical limit, you'll see an error message when you try to follow. Remember that you cannot follow a user who is blocking you. We do not limit the number of people who can follow you.
If you've reached the account-based follow limit (2,000 users), you’ll need to wait until you yourself have more followers before you can follow additional users. Follow limits are system-wide; Support cannot remove or adjust your follow limits.
To follow one or two additional users, unfollow a few accounts you're currently following. Please note, however, that regularly following and unfollowing many accounts is a violation of the Twitter Rules and can result in account suspension.
These limits help us improve site performance and reliability and help us make Twitter a nice place for everyone. We’ve included a more in-depth discussion of why we have follow limits on the Follow Limits and Best Practices Page.
Some API administrators have whitelist status so that their applications can function without hitting system limits for direct messages and API requests per hour. Whitelisting does not increase the follow limits and all accounts are subject to the same follow limits and rules. You can find information on our current Update, DM, and API Requests on this About Twitter Limits help page.