Hover state doesn't work on slider with custom styles

I have defined a style "sliderBarStyle" for the slider-bar, and a style "sliderStyle" for the slider-thumb. The bar and thumb get drawn correctly, and work correctly. But the hover-state never seems to activate. I have set a background-image for the normal state, and a different one for the hover state, but it is always the normal-state image that is shown.

I use it like this: newDistanceX = GUI.HorizontalSlider(ColumnsSliderRect, newDistanceX, 0, 1, sliderBarStyle, sliderStyle);

Any idea?

Edit: I've also tried it with the other states (focused, active), but it's always the normal-image that's beeing shown