My childhood doctor in the late 70's hated insurance companies over this. I'd hear him on the phone arguing with the insurance company reps asking if they had a MD Degree and would like explain the denial to him. Interesting to hear back in the day.
https://www.youtube.com/watch?v=Vp7u58R41N8&list=PLpMVXO0TkG...
I know my sister at one point was having some critical medication for her denied so the doctor just kept giving her sample packs.
I went through an issue with a medication for me and it was still never resolved.
Insurers may also require step therapy. Try a cheaper treatment first, then if the patient fails to respond they will authorize a more expensive option.
I went through multiple appeal processes, my doctor tried to file multiple on my behalf, I called, and nothing.
They don't care.