Twitter has a Help Center Article that explains why some people are not able to follow new people. You can read the full article at CLICKING HERE.
Twitter has imposed reasonable limits to help prevent system strain and limit abuse. If you hit a technical limit, twitter will tell you by showing an error message in your browser when you try to perform an action.
Technical Follow Limits
Twitter does not limit the number of people that can follow you, but twitterhas put limits on how many other accounts you can follow.
Every account can follow 2,000 users total. Once you’ve followed 2,000 users, there are limits to the number of additional users you can follow. This number is different for each account and is based on your ratio of followers to following; this ratio is not published. Follow limits cannot be lifted by Twitter and everyone is subject to follow limits, even high profile and API accounts.
In addition to this account-based limit, every Twitter account is technically unable to follow more than 1,000 users per day. Please note that this is just a technical limit to prevent egregious abuse from spam accounts. Accounts are also prohibited from aggressively following other users. Our Follow Limits and Best Practices Page has more information on Twitter’s following rules.
Love this from the Best Practices Page:
So how am I supposed to get followers? Remember, Twitter isn’t a race to get the most followers. If you follow users that you’re interested in, it’s more likely that legitimate users will find you and read your updates. People follow other users on Twitter to read updates that are interesting to them. Aggressively following and un-following accounts is frustrating to other Twitter users, and degrades the Twitter experience for everyone.