Users are getting Delivery Locked against their own ID

Hi,

Our business users are getting message as delivery locked against their own ID only during midnight for 2-3 hours.

So they cannot change it in VL02N T code.

We noticed in no other screen delivery is opened by any other user .Also noticed no entries exists in SM12 and SM13 against business user ID.

So could you please us know what could be the possible reasons for this locking issue.

Regards,

Praveen