You have two interactive objects (Next buttons) pausing the slide at 1.5seconds and the second one a little bit later. Apparently you didn't check the option to pause the slide audio at that pausing point which means it will continue but all assets with a timeline starting after those pausing points will not appear until the playhead is released. That can only happen when the button is clicked and you allow the playhead to continue. Move those pausing points to happen after the start of the last object timeline which will be around 20secs.
Lack of understanding Captivate's Timeline is the number 1 stumbling block, resulting in many questions like this one on all forums. Please take some time to learn about the Timeline. Here is one possible link:
http://blog.lilybiri.com/pausing-captivates-timeline
BTW Play Slide is NOT a preview method, although it appears under the Preview button which is IMO a real UI bug. It is only meant to be used to edit slide timings, but will never give you a real preview. I pleaded to get it out of the Preview menu, but the team only agreed to change its name to 'Play Slide' instead of Preview slide. It mimicks what happens when you click the Play button in the Timeline control panel.
Real Preview at this moment for a non-responsive project which will be published to HTML is only possible with F11, Preview HTML in Browser. You would have seen the situation as close as possible to the final output.
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