Hi,
If you haven't already, click on the 'Fit' button under 'Tk 2d UIItem > Collider > Automatic Fit' which should set the collider size to the same size of the button.
If this doesn't work, it may be the collider for the button is largely 'behind' another object or collider, try moving the button to a different area of the screen to see if that detects it. Or it may be that the button's sprite also has a collider enabled on it, which is eclipsing most of the collider on the UIItem. If that's the case then disable the sprite's collider.
If none of that works, can you post a screen shot with the UIItem button in scene mode?
Thanks.