Do you have the Pages panel open? Which page is selected when you are working on the file adding your content? You should have the master page selected to add content to the master page (which in turn will appear on all pages based on that master), but you need to select the individual document pages to add content that belongs only on them.
Can you show us a screen shot of the Export dialog, please? I'm not sure exactly what you are exporting. Use the camera icon on the web page to embed a screen capture like this:
I keep selecting the document page, then when I try to add a text box, it goes back to the master:
I'm trying to export to PDF (which I'd never had trouble with until I started trying to setup master docs)
Maybe I'm just doing this all wrong. I'm trying to set up two templates, each of which is the same design but tweaked so that one is for A4 and one for letter size printing.
I thought I'd try to make everything better-set-up by setting up masters (containing my footer and some logos) and layers, but even though I've watched a load of tutorials on those and other setup tips online, I can't seem to get it to function correctly.
One of my frustrations is that I've been using InDesign for 1+ years but this is the first time I've set up a template on my own and so I feel like I "should" know how to do this...
Your first screen shot shows that you have selected the master page. You must double click on the document page to select it.
Thanks, I'm such a idiot, I was only single-clicking.