Hi Thomas,
this may be a timing problem.
When a link on a switch becomes active it will generally take around 40 seconds before the switch actually passes traffic over the port due to the spanning tree protocol, so it is possible that the PC has already sent its DHCP request when the switchport was not yet active.
For PC's directly connected to the switch, you can configure their switchport with "spanning-tree portfast". This shortens the time between the different spanning tree stages, so never do this on port connected to a hub or another switch.
hth
Herbert