Some of our ASA's won't accept new username/pw combinations.

We have a ton of ASA 5505's out in the field and we're trying to update our credentials on them. We are using local credentials, no tacacs or anything. When I run the command to add a new username, it accepts and I see it in the config, but it will not take the new login. I've tried typing and copy/paste. Changing the enable password works like a charm. Any ideas?

conf t username tester password testerpassword priv 15 

edit: some of the asa's will accept the new creds and some won't.