Make sure that the PC’s NIC is set for DHCP in the network configuration. Also make sure that you have some free addresses available in your DHCP range. If you have 30 computers, and only 25 addresses then you would have problems. Also, most Routers will not try to assign an address via DHCP if there is already a matching static address, but best practice is to make sure your static addresses are outside of your DHCP Range.
Previous answer overwritten:
Could be the switch ports are not negotiating link properly. Set the ports to full duplex, 100Mb/sec. Another possiblity is that the ports are members of a specific VLAN and not forwarding DHCP requests properly.