Troubleshooting ETRN

What are some ways that I can troubleshoot an ETRN problem? I am using Exchange 5.5 and I am not getting mail from my ISP except mail that is being sent to me at the same time that I am connected to the ISP. I have Exchange set to dial-up my ISP and get my users mail via ETRN. Mail that was sent to my ISP mailbox before I connect is not being downloaded to Exchange. Any suggestions?