Featured Slider disappears

I use the "Featured Post slider" with 2 posts/images. I also set the height of the slider to 220px via the following code:

.featured {
	height:220px;
}

.featured .img-effect {
	height:220px;
}

At first, the slider shows fine on my website, but after a while it disappears. When I update the settings in the layout menu, the slider re-appears on my site...but then it disappears again after a couple of hours. Is there any explanation for this strange behavior? The site is: http://www.scrapbooking-karin.be/newwordpress/