Next button greyed out till timeline completes

I have just started work on a Storyline project largely built by someone else. I am using Storyline 360.

This is using the classic player.

On all slides the Next button is greyed out until the timeline finishes, yet I cannot find any triggers creating this effect. 

Searching through this site there is talk about restricted content navigation which I thought might be causing this, but I cannot find where this is turned on or off.

Can anyone shed light on this?

Drew