AirPods wireless charging case flashing amber on charging pad. doesn't charge

I just got AirPods with the wireless charging case. They charge fine plugged in but will not charge on my Mophie charging pad that I have used for my iPhone 11 for about a year. The case light flashed either fast amber or a slower amber. I can't find any guidance on this other than the possibility that the charging pad somehow isn't compatible, even though it should be.