UI Buttons does not invoke directly listener function on mobile

Hi,

i am using unity 5.4.0 and i am developing a game for mobile. On the mobile device all buttons in my application does not immediately invoke the underlying listener after a click. However, i can see that the "pressed" state of a button raises up and the graphic changed, but the function (a simple method call which stays in a monobehaviour) will not be immediately invoked. After several times hitting that button with the finger it works.

Does anyone have a clue or hint for me what i can do to improve this behaviour?

Cheers, Chris