Apple Pay has stopped using my Apple Cash

I had a large balance of Apple Cash and when I ordered an item online and used Apple Pay it defaulted

to my debit card and not the amount in my Apple Cash. I lose 50¢ every time I have to send my Apple Cash immediately back to my bank account to cover what Apple Cash didn’t.

How do I get the default back to Apple Cash? If I can’t, why have Apple Cash at all? It costs me money.