TV Subtitles cutting out after a short time, any ideas?

[Master Of Puppets]

My mum relies on subtitles these days but she has been complaing about certain channels that only show subtitles for a minute or so, then
cut out, she has to flick channel and back again for them to reappear but they just cut out again shortly after.

The worst channels for it are BBC, ITV 3 and 4, and channel 5, most other channels are fine.

I've had a look online but nothing specifically relating to cutting out after a short time, I don't suppose any of you guys know how I could help her out?

[Junglebert]

What tv/streaming box/etc is she using?

[Master Of Puppets]

Samsung TV and Freeview, very simple setup.