Family member unable to use credit in family sharing

I set up family sharing with myself as the organiser and my mother as an adult (she doesn't want to use a credit card but she is happy to use iTunes cards to put money in her Apple ID account). There are no other people in our family group. 

She has money on her Apple Account and it shows it when she wants to purchase something (just a single song for herself and not as a gift). The payment methods on her phone are: Apple Account (with plenty of money in it) and my credit card is next.

Even though we can see it we are unable to select the first option when she wants to purchase something and then it wants her to verify my credit card details. I tried to add money to my own Apple ID so it would use that instead of my credit card to see if that makes a difference. Now if I purchase a song on my phone it uses my Apple Account Balance first but it doesn’t make any difference to her options (at this point I’m grasping at straws and hoping for the best).

I can’t find any way of fixing it and the only thing I can think of to try next is to remove her from family sharing and add her again. 

My biggest concern is I use an app that she originally purchased. It was a one off payment and now it’s moved to a very expensive subscription model. I know I can use the app as it was originally purchased as part of the family sharing but there is a lot of information in it (uni notes and other projects) and I’m concerned I will lose that information if I disconnect her from our family group and lose access to the app and re-add her. 

This has also happened with another family member who decided it was too hard to deal with and left the family group. Is there any way of fixing this so she can use her own Apple Account Balance? It doesn’t always happen but we also can’t see any pattern for this to happen.