From theses subtitles:
I no longer have my powers.
Well, what happened to them?
Oh, I don't know, Luci, pick a sin. Is it because I released a damned
soul from Hell? Slept with a demon? Is it because I'm working with
you? Or because of Mom? Or am I simply no longer worthy? (sniffles)
It's time that I faced the truth, brother. I've fallen.
Well. Welcome to the club. Meetings are on Tuesdays.
It's different for me, Luci. It's different. I no longer have my
powers. My very strength...
Big deal! It's probably the best thing that ever happened to you.
We don't really know why, but there could be various reasons as he stated : demon affair, helping Mom, helping Lucifer, Mom interference, etc.
In this episode, they said also that no-one really knows what want God, and what he does really, and why is someone worthy or not.
For the point of Lucifer, Amenadiel was always very powerful (that was his "gift", while Uriel's one was "prediction", Lucifer knows desires, etc., he simply was more powerful), so he now understands what's being over-powered, defenseless.
Also, he's fallen, as Lucifer whom was casted away from Heaven and had to rules over Hells. He's no longer help by God, he's alone, like Lucifer. But also, he doesn't really care, he said "Big Deal", Lucifer already faced that from a long time, and he has urgent matter (saving Chloe from Uriel).