When revisiting a slide, doesn't work like I'd expect it to

For clarification - what behavior are you expecting to see?

• User revisits the slide and plays it a second/third time
• User revisits the slide and skips through it with the playbar
• ...?

Knowing what you are expecting might help.

If you're going to show the seekbar but want to ensure the user views the entire timeline, set it to "Allow drag after completion."

That setting prevents the user from dragging the seekbar until the full timeline has been completed. Here's the User Guide article. 

Unfortunately, even though the user can drag the seekbar after they've viewed the full timeline, the built-in replay button doesn't work. To replay the slide, the user would have to drag the seekbar back to the beginning and then click play. 

An alternative is to add your own Replay button on the slide. That could be hidden until the timeline ends.

The attached file demonstrates both options on the same slide. The Slide Properties are set to "Resume saved state."

FYI, according to the User Guide, this is how Storyline "automatically decides" what to do when revisiting:

• If the slide has only simple objects—text, images, audio, video, etc—but no interactivity, Storyline 360 resets the slide to the beginning of its timeline.
• If the slide contains interactive elements, such as buttons or other objects that have a visited/selected state, Storyline 360 resumes the slide where it left off previously. 

 

Here's what I recommend for the “When revisiting” setting in Slide Properties:

• Keep "Automatically decide" for graded-question slides. Storyline will know whether to reset the slide (when the user needs to answer the question) or resume it (when reviewing).
• For other slides, change the setting to ensure it does what you want.

The "replay" function within the slide is designed to allow users to view the slide again while they are still on the slide - before advancing to a subsequent slide.

The slide properties options control the behavior for revisiting the slide, after advancing to a subsequent slide.

Given that you want the user to go back to a previously viewed slide, you probably should select "reset to initial state" or "automatically decide." But with locking down the seekbar, you are essentially forcing the user to watch the entire slide again, regardless of whether they want to see the entire slide.