Animate - Animation is previewing correctly but only displays top layer when exported.

Hi guys,

I'm just making a simple 21s UI animation which just uses .jpeg files. It previews perfectly but whenever I go to export it (H.264, YT 1080p) the final .mp4 is just 21 seconds of the top layer with no hint of the other layers. Wondering if this is anything to do with hiding layers as a blend object up until a certain keyframe?