Hm, are you sure they're an account you muted before? Sometimes spammers use the same name, but their account is actually different (the "npub...")

Otherwise, you can use mutable.app to check your mute list and manage/add/remove accounts there manually.

I tend to use jumble in the web these days as I host it myself, or nosotros.app.

For mobile apps I am fairly happy with Amethyst, which I feel has the best balance of proper supported features and usability. I know some other mobile clients look and feel "shinier", but none come close to Amethyst in terms of support and reliability I've found.