There's a number of reasons why tweets from within the game may fail to post but here are some of the most common:
Intermittent internet connection causing the connection to fail.
Incorrect date/time on device causing Twitter to reject the connection.
Attempting to post the same tweet multiple times within a certain time limit (i.e. multiple manager of the month awards) – Twitter refuses duplicate tweets.
If none of the above are valid, please feel free to let us know via the forums and we'll look into things.