I’d recommend to update to 10.7 and use the latest user_ldap app version available. There have been some bug fixes around ldap sync, although I’m not sure about the exact version, so your problem might be fixed.
In addition, please provide logs so we can have a look at what’s happening. We might also need your configuration (there are some known issues due to “wrong” configurations), and maybe some info about the ldap users in case there is something weird with them.