Q: Macbook External Touch Screen error

Hi All,

I am using a Macbook Pro retina 15 with an external touch screen, in an extended desktop. Sometimes when I connect the screen, the touch interface works fine, other times when I touch on the touch screen it puts the mouse on the macbook screen. In other words, anything i touch on the screen affects the internal macbook monitor and not the second screen. How do I assign the touch control to the second monitor? Really frustrating as there is nothing on the web as to how to do this, especially as sometimes it's fine, other's not.

Please help me out ASAP, I'd be forever grateful!

Cheers

Paul