[otrs] No tickets in customer frontend

Hi list ! We are using OTRS 2.0.4 on debian and facing the following issue. Some customers can see their tickets via the customer web frontend and others aren't seeing any at all. 1) What can be the reason for that behaviour ? 2) How can I simulate the SQL query OTRS is doing here so I can perform the query manually and see the differences in the mysql DB. Best regards Sven