Can't start clockwork recovery image lately
Try flashing the Recovery image manually via fastboot, as described in this guide:


  
  Download the latest version of the ClockworkMod Recovery from here.
  Place the ClockworkMod Recovery Image in the same folder as fastboot (the /tools folder with the Android SDK folder).
  Power the Nexus S down, and hold Volume Up &amp; the Power button until booted into the bootloader. You should see a white screen with
  three skating androids.
  Connect the Nexus S to the computer via USB. On the computer, open terminal and run: fastboot flash recovery recovery-clockwork-5.0.2.0-crespo.img
  
  
  The ClockworkMod Recovery should now be installed on the Nexus S.