sending text to non iPhone users from my Macbook

That message will literally be forwarded through your iPhone and over the cellular network.

To have any hope of it working, your iPhone must have a cellular plan, be powered on, and be in range of a cell tower.

It's still not perfect. The Trade Press have been bashing Apple for unreliability lately, without any specific explanation of why it is not as reliable as it should be.

<< I hope they can address this >>

Apple is not working on this as their issue.

To be heard, You would need to contact support and give them enough specific information to do some debugging.

in the meantime, when it says "not delivered" I suggest you send the message again. If your recipients are like mine, they will often report the message received multiple times.