Ah! I think I'm part way to finding the problem, I'm printing the pointer position out and:
My desktop is 1920x1080 and if I move the cursor all the way to the bottom right of the screen it's saying its position is 1920x1080.
The tablet that works is 1366x768 and if I go to the bottom right it says its position is 1366x768.
Now on the tablet that isn't working: it is also 1920x1080 but if I move the pointer to the bottom right...it says it is 1366x768.
So there is something strange going on here where the DirectX DXSwapChain is 1920x1080 (as I get a 1920x1080 screenshot if I use the DirectXTK SaveWic...() function) but the actual windows app itself thinks that it is 1366x768.
Does anyone with windows store app experience know how this could have happened? Why does the pointer think its range is only 1366x768?
Thanks for your time and the help so far!
Edit: I've just taken a print screen on the 1920x1080 device and it saves as a 1920x1080 png...so the app thinks it's 1920x1080, directx thinks it is 1920x1080 but the pointer framework/library thinks it is 1366x768 (which the other tablet is but my desktop isn't).