Help with variables and the master slide

Hi,

I've got a project with a few scenes with a custom player and I'm stuck.

On the master slide there's a player layer with buttons for the menu slide, back and next.

The various variables and triggers are present to display the player layer from the master slide and to activate the back, next and menu buttons.

The player layer first appears on slide 2.

The next button works on slide 2 to go to 3, but for some reason it won't work for slide 3 to 4. If I click the Menu button, which also displays slide 4 and then go back to 3, it works. 

I have no idea if I'm missing something obvious.

All help and suggestions gratefully received.

I've uploaded a cut-down version with just some placeholder text.

Thank you