There are a number of reasons that could cause your following to repeatedly fail. One possible problem is that twitter is down. This happens less and less these days, and a more likely scenario is that you've hit your 2001 follow limit.

Twitter will only let you friend up to 2001 people while your own followers are below this number. When your own following get's to this point you can friend 10% more people than you have followers. This is a measure they've put in place to stop spam. So, in order to be able to follow more people you'll always need to make sure your own follower count is up around the same number as your friends.

This obviously shouldn't occur if your friend count is below your follower count.