When using ASP.NET membership, you should always be sure to specify the applicationName attribute on all providers. If you omit it, and deploy your database to the web server, you will find, that all your users have vanished (or at least cannot be found). So make sure your web.config looks similar to this:
<membership>
<providers>
<remove name="AspNetSqlMembershipProvider" />
<add name="AspNetSqlMembershipProvider" applicationName="/" type="System.Web.Security.SqlMembershipProvider, System.Web, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" connectionStringName="LocalSqlServer" />
</providers>
</membership>
After setting the application name, you can start adding users. If you added users before specifying it, they will probably no longer be available.