A slider element is present, but you don’t see it functioning (in the editor or in preview mode).
- Verify that its start and end times are correct.
- Verify that its breadth and color are visible.
- Verify that an element (even a blank one) isn’t in front of the slider. If this is the case, move the slider to the foreground (via the timeline or via the menu « Arrangement > Foreground »).