Storyline 360 - Keyboard nav not working

I am building a course in Storyline 360 (v3.75.30269.0) and I want it to be WCAG AA compliant but I cannot get my keyboard to focus on all the objects in my slides.

I have hidden all default navigation controls and created a customised navigation ribbon which sits in a layer that is set to show when timeline begins on base layer.

When I try to navigate the screen using a keyboard I cannot access any of the base layer text boxes. The only object the keyboard will focus on is the Start course button.

I have tried re-ordering the base layer items in the Focus Order settings so they appear at the top of the list, before the items in the Navigation layer, but this doesn't change anything

- Why is this happening?
- Is it something to do with the navigation layer showing? 
- How do I fix?

Please help - many thanks,

Jess