Windows 10 – Messaging Software

I apologise if I am doing something just very silly but I am struggling with this messaging programme in Windows 10.

I will set the stage.

I have a Surface Pro 3, with a fully up to date version of Windows 10 on it.

I also have a Lumia 970. Again up to date.

Both are logged in with the same Hotmail account.

The phone does everything I expect it to so I don't think the problem is there.

But on the Surface the messaging programme doesn't seem to work at all. It has some of my conversations in there, but only conversations from before March. And it wont ever send a text message.

If on the surface I just use Cortana to send a text, that works fine. It sends it to the phone and it is sent.

And when I receive texts they all come up as a notification on my surface.

So then why can't I then reply from the surface using the messenger programme? Why is it not saving or synching them properly?

Any advice, a set of options I haven't ticked properly?