Cannot reconnect all network drives

Running XP Pro. I have 6 mapped network drives. When I
boot up the computer, I get a message in the lower right
corner of the screen that reads "Cannot reconnect all
network drives". It does this most of the time, but not
always.

Then I need to open Windows Explorer and click on each of
the drives to "activate" the connection.

Any idea why it just doesn't connect on boot up? Why do I
need to mannualy click on the drive to establish the
connection each time?