[Zope] creating new users

We've had two installs of Zope 2.4.3 on a Sun Solaris and a Linux server. In both instances, Zope is not recognising new users that we have defined for a folder in the normal way (i.e. via the acl_users folder). It just doesn't like the username or password that we try and enter. After three attempts, it errors. At the moment, the only account that is allowed by Zope is the admin account created during the install. Can anyone offer any suggestions? This has happened on two different servers, with different OS's but with the same version number of Zope. Thanks in advance!