DNS Zone Transfer Failure to Secondary

We have 7 forward lookup Zones, a Standard Primary and Standard Secondary DNS Servers (Win 2K). Recently the zone transfers from the Master to the Secondary have started failing randomly for the first 2 zones only. It seems to happen at random intervals. Here is the error…
Event Type: Error
Event Source: DNS
Event Category: None
Event ID: 6527
Date: 25/08/2002
Time: 11:30:58
User: N/A
Computer: Whatever
Description:

Zone whatever.com expired before it could obtain a successfulzone transfer or update from a master server acting as its source for the zone. The zone has been shut down.

However the other 5 zones are not experincing any problems. John