Views Slideshow doesn't work in custom theme

I'm using a custom theme based on Zen with a slideshow view in a sidebar and although the block for the slideshow shows up, the slideshow isn't working. Only the first item appears and transition to next item never happens. This is the case for i.e. and firefox. It does work wonderfully when using the Zen base theme as well as other themes, but obviously something is missing from my custom theme. Can anyone tell me what component my theme is missing?