Apple Watch SMS issue on cellular

I recently bought an Apple Watch (S4) and got the cellular plan with it as I liked the idea of being able to leave my phone behind and not miss an important call or text.

So far it’s been working great but today when I was out on a walk I got a call from a friend (which came through perfect on the watch) who referenced a text he had sent a few min earlier. Alas, no text on the watch. This was a green text from a non-iPhone user. (I have not experienced any issues with iMessage.)

When I get home and check my phone I see his text. My phone was on and connected to cell/WiFi the whole time. I disconnect my watch from my phone (turned off BT and WiFi) and ask him to text me again... comes in on the phone but nothing on the watch... I try and text him from the watch and after displaying an error at first, it goes through and he confirms receipt.

Is this just a quirk of the watch/my provider, or is there anything I can do?

Thank you kindly