If you are having difficulty sharing someone else’s Tweet with your followers via Retweet (RT), it’s most likely because their Tweets are protected.
To verify if the user you are following has protected their Tweets, visit their profile page by clicking on their avatar or their username next to their Tweets in your timeline.
If you see the lock icon next to the user’s name and information on their profile page, their Tweets are protected and you will not be able to share their Tweets on your timeline through any Retweet or RT feature, on twitter.com or in a third-party application. If you ask them nicely, they might make their Tweets public so you can share them with your followers.
Were you trying to Tweet from a list view? Currently, Twitter’s Lists feature does not support the Retweet function within lists. If you hover over a Tweet in your list, you will notice that you have the option to reply to the Tweet, but in order to retweet it, you must visit the user’s profile page as shown above and retweet manually.