My friend and I were going for the same username using Namemc. It tells us when the username is ready, but whenever I put in the username and wait until it's ready, it doesn't refresh and I have to re-paste the name in for it to be available to me. But whenever my friend does it, it automatically refreshes for him and he can click "Change Username" without re-entering the username in.
Can you provide the device and browser of each device?