Make sure you connect to the correct LUN on the iscsi target. See sanuri for the correct URI syntax for specifying the LUN.

Really read the above, and make sure that the LUN number is correct. It is the actual LUN # and not index of client accessible LUNs.

See this forum post about a common confusion about the LUNs number.

Use tcpdump or Wireshark to check the iSCSI traffic for what is going wrong. Build iPXE with DEBUG=iscsi might give more details, but a packet dump is likely more useful in this case.