Q: How do I get the same messages to show up on my Macbook Air and iPhone?

I've already done that thing where you enter the passcode from your computer onto your phone to set it up, but it just isn't working. I receive most messages on my phone only, and I cannot access them on my laptop. But on the other hand, there are a few specific contacts that will send me a message and I won't receive it on my phone but only my computer. How do I fix this so I can see my messages on both devices?