Captions problems: small and misplaced when full screen

Hi everyone,

I'm having an issue with Teletext captions. When in preview mode there's no problem and they appear as I've placed them like this:

However, when I playback in full screen, the captions appear like this:

They're closed captions (Teletext Level 1). I've tried to preview them as an .stl file in VLC to see how they export; VLC is importing the file fine but the captions aren't showing at all - not sure if this is the same problem or a second one. 

Anyone ever seen this before? Any advice would be very much appreciated, thanks!