Yep, went back and looked... I am also using the Touch GUI Event. As 4ppleseed mentions, quite odd that not touching is only available in the Touch GUI event.
Though, maybe its me, i cant seem to get Touch GUI events to work correctly in Unity 5.
Edit -- I did get Touch GUI to work, I just forgot I need to "Make it visible" by having its screen position set somewhere between 0-1.
I think AllorNothing's suggestion is probably the best, as you can easily scale the object under the players finger. Though it would require creating an object for each touch. That way you can drag a finger onto the button and have it trigger and stop when its slid off.
not 100% sure how to set up in playmaker/unity, but I solved the same problem in another engine by spawning a small object at the touch position, and then monitoring the collision between the touch object and the button. Releasing touch destroys the object.
Should be simple enough to implement in playmaker, would that work here?