Why do some text on my iphone show up on my ipad and others to the same person don't?

Some of my text messages sent from my iPhone will also show up on my iPad and others to the same person don't. Also it seems that only messages to two people are showing up on both devices yet I also have quite of few friends that also have iPhones and my text to them never show up on my iPad. What determines why my messages show up on both devices or not?