I see this happen quite often so I hope I can help at least a few people by clarifying this. Recently, Twitter changed the option we once had to see all @ replies even if they were sent to someone we weren’t following. Now, if you send a tweet that you want all of your followers to see but post something like, “@Starbucks is giving away free coffee”, the only people who will see that tweet will be the ones that follow both you and @Starbucks.
If you send a tweet or a reply to someone and you want all of your followers to see it, you need to put something in front of the @ symbol. So, for example, if I wanted to let all my followers know that Starbucks was giving away free coffee, I would tweet, “Wow, @Starbucks is giving away free coffee.” As long as something is in front of the @ symbol, it will be visible to all of my followers. If you want to save characters you could always use something like, “> @Starbucks is giving away free coffee.”
Disclaimer: To my knowledge, Starbucks is not giving away free coffee today.