If the client is not able to boot from the target server and the problem is not discernable from the immediately available information, you may need to use a network monitor that is capable of parsing the iSCSI protocol to troubleshoot the information.
If the iSCSI target is software running on a server, you may be able to run a software network monitor such as Ethereal directly on the target system. Otherwise, you may need to configure a network switch with port mirroring and direct the mirrored packets to a port where either a hardware or software network monitor can be attached.