I am sorry to hear this. My last guy died. Same thing. She was shy and would not leave her cave. This time I added things in the cave to disturb it when I feed. I also have a pair of very aggressive eating clowns. I think Regal Angels are smart and learn to eat by watching other fish.
My baby Regal is a very aggressive eater now. I try to feed her dry food instead of frozen food. IMO, dry food are much more energy dense and have much wider amount of nutrients than frozen food. I am glad my Regal prefer to eat flakes over frozen. Here is a video of him eating just now. This is the 4th feeding today. I am home so I feed them every few hours. Only two feeding on weekdays.
https://www.youtube.com/watch?v=FpbNF0HFPtA