Depending on what switch you're plugging into, this could easily be Cisco Portfast or similar catching you out.
When the port goes active (layer 2 link) the switch inspects but doesn't forward packets to try and ensure you've not just added a loop into your ethernet network. After a minute or so, if everything looks normal, the switch then starts letting the device talk to the rest of the network.
If you don't know about it then this can be annoying at best as you seem to have a layer2 link but DHCP can seem to take an age to start working :)
When the port goes active (layer 2 link) the switch inspects but doesn't forward packets to try and ensure you've not just added a loop into your ethernet network. After a minute or so, if everything looks normal, the switch then starts letting the device talk to the rest of the network.
If you don't know about it then this can be annoying at best as you seem to have a layer2 link but DHCP can seem to take an age to start working :)