Mac failing to recognize username & password

We have a problem with our Mac. It fails to recognize the username/password for the other person's account. What I mean by that is that if I'm logged into my account, and I want to install something or download updates, and it needs authentication on both my account and my wife's (the "other person's account"), it will accept my credentials but reject my wife's. Likewise, if she's logged into hers, it will accept her credentials but reject mine.

What could be causing this and how can it be fixed?

You need to log in, run the update, log off and have your wife log in and run the update again. Most applications can be updated from any admin log on and will apply to all. However, there are some that can not because of the way they were installed. When installing software on the machine that both you and your wife use, if the application gives you a choice, select install for all users.

Thanks chscag,

I *think* that worked.