Oh! I have a 50 year old who can’t say “v” and says “th” instead. A mystery! Harder to teach an old dog new tricks though… 😉
I also have your exact issue with a student my age – I think it’s her Kent accent! So apparently I’m struggling with the adults!
I’ve used the same methods as you and it can work when they’re thinking about it. Sorry to not be of further help but I imagine Matt Pocock might be, if he reads this. Would be interested on people’s thoughts on this as I imagine it’s more about elocution/speech therapy which I think Matt knows more about?