[Serusers] problem establishing conversation between two sip users

We are now running ser 0.8.12 daemon on Solaris 2.8 platform without persistency support. When we do the testing, we tried to establish a conversation between two X-lite 2.0 clients built on Mac OS X. After we started the X-lite, we configured it to use our sip server. The user can log into the sip server properly. However, when we tried to initiate a conversation from one X-lite to another, even though both of them are logged into the same host, they could not reach each other. The invitaion finally timed out. We also tried to initiate a conversation from X-lite to Windows messenger. It works fine, but we could not initiate a conversation from Windows Messenger to X-lite. Do anyone have similiar experience? Is there anyway to check sip users' activities on the ser side? How do we know whether there is communication between two users? We tried to use "serctl moni" to monitor the activity, but the log is long and confusing. Is there any documentatiion that we can refer to? Thanks in advance.