iMessage always sends texts

I have an iPhone 5 with iOS7. I used to have the typical problem of it starting to send an iMessage then stopping right before the end and just not sending the message at all. I haven't had that issue lately. However, the problem that I am having is that iMessage is sending texts all the time instead of iMessages. Even if I have full signal.

I have a friend (friend A) that had an iPhone, switched to a normal phone, and is now back on an iPhone. He is able to text me with iMessage, and it always comes to me as an IMessage, but whenever I try to respond, my phone immediately just sends him a text instead of an iMessage.

Another friend (we'll call him friend B) used to have an iPhone and now is on a different phone. Whenever I try to text him, my iPhone tries to text him, it always tries to send as an iMessage instead of text.

These are just 2 examples, but I'm not sure exactly what is going on with my iMessage. Any ideas?