Delay in display of "Downtime Notifications"

Hi,
We have scheduled a downtime notification by using the following steps:


1. Login to OAM as administrator and create a downtime schedule as follows.

1. Click on Sitemap -->  Maintenance --> Maintain downtime schedules
2. Click on Schedule Downtime link    
   

2. Enter the details in the page and click submit

Issue:

Once i set this notification to be displayed at say 00:00 hours, the message is getting displayed only after 15-20 minutes (of the set time, ex: 00:15 or 00:20) and sometimes even takes a maximum of 45 minutes(00:45).
The issue is intermittent and the time varies.

Is there a background process to be run, such that the notification is displayed at the right time?