First mouse click on buttons does not display 'pressed' image

Hi, does anyone have an idea why the first mouse click on a button would not display as ‘pressed’ (i.e displaying the pressed image does not work, while the ‘on pressed’ event works fine)?

I have to press my button twice for the ‘pressed’ image to show up, once I’ve pressed it twice the button lights up correctly every time when clicked until I press something else.

Does anyone know why this might be?