When you have a large data set and a record triggers a notification that sends an email when a BIRT report link is attached, if the query takes too long to retrieve results, users will not be able to see the report. The session will expire before the email containing the link is sent. For example, click on the link to a BIRT report in the email notification, and you will receive a message stating the session has expired. Other messages have been observed, as well as the request to sign back in.
By default, a report is written against the active session. So, if the session times out, the report is lost. To resolve the problem, you may check the “Queued” box in the Options tab of the report. That way, the report will run in the background and it will write to the user directory rather than to the session.