Hey there @ricinport! Sorry I didn't see this message when you originally posted it. Sliders are funny things. Most times they need to be able to "see" the page during the loading process. As the page loads the slider looks to see how much space it has available to it so it can decide how to size things. When it is in a modal popup, like a lightbox, it is hidden during the loading of the page. So when the slider takes a look to see how much space it has available to it, it sees that it has none -- literally zero space to work with. So it sizes itself accordingly... this unfortunately means it has a height of zero.
This goes for Reveal as well -- Reveal hides its content until it is unveiled later as the user scrolls down to the content. Since Reveal's content is hidden the slider thinks that it does not have any space in which to display itself.