Welcome back to the Disney Parks Mom Panel, Jennifer! Thank you so much for this great question.
I completely understand your frustration trying to make FastPass+ selections to see Anna & Elsa. I am also having no luck securing a FastPass+ to see my favorite royalty from Arendelle.
My first suggestion, which seems to be what you are already doing, is to keep trying. People tend to change their plans as they get closer to their vacation and you never know when one might pop up. The next suggestion is to arrive at the Magic Kingdom early and head straight to Princess Fairytale Hall. Lastly, you could arrive early but head straight to the first
FastPass+ kiosk and make your selection for later in the day.
Whichever method you chose I will be sending pixie-dust your way! If you have any other questions please come back and see me.
Wishing you a magical vacation!
Sue N., IN