This will be down to the broadcaster. VM will always forward the subtitles if they are available in the incoming feed.
Is the first showing live? Adding subtitles to live broadcasts is considerably more difficult than adding them to pre-recorded content.
It's also possible that the repeat showings aren't exactly the same as the first broadcast. Broadcasters create different edits of the same programme for timing reasons, and typically they will only subtitle one of the versions.
========= I work for Virgin Media but all opinions are my own