Placeholder content switching when moving between layouts

Thanks Jose, I found the answer. 

I discovered that; 

1. Title text placeholders are different from regular text box placeholders (image attached to show how Title placeholders are identified). I'd inadvertently removed the Title placeholder, and couldn't work out how to insert a new one, so I copy/pasted one from another project into the parent slide, and that has worked. 

2. It's best to copy/paste text boxes in the slide master, from one layout to the next, and adjust placement, size etc. rather than creating new text boxes.  This way the content transposes the content from text box to text box correctly. 

Sounds obvious to me now. Lol.

Thanks.