How to use AirPods earbuds with a different case?

My friend has a pair of AirPods Pro 2 earbuds, but lost the case. And he has another AirPods 2 Pro case for which he had lost the earbuds. Both old and new were linked to his own Apple Account. So, he went to the obvious solution: Use the existing earbuds with the existing case. But he reports having had "trouble". Is there something that a user needs to do to be able to use AirPods Pro 2 earbuds with a different case?