Can't start clockwork recovery image lately

"Lately" (like in the last few months), I'm unable to start the Clockwork Mod recovery image on my Nexus S with [Oxygen 2.2.1](http://forum.oxygen.im/viewtopic.php?id=165) . When I select "boot to recovery mode" (or want to start a ROM backup) in [ROM Manager Premium](https://market.android.com/details?id=com.koushikdutta.rommanager.license&feature=more_from_developer#?t=W251bGwsMSwxLDEwMiwiY29tLmtvdXNoaWtkdXR0YS5yb21tYW5hZ2VyLmxpY2Vuc2UiXQ..) , it reboots and I see the Android icon. The very first line is red and reads: **FASTBOOT MODE - NO BOOT OR RECOVERY IMG** I've (supposedly…) reinstalled the recovery image multiple times and ROM Manager tells me, that I'm on the latest version already (5.0.2.0). Well… How do I install the Clockwork Mod recovery image? My phone is rooted, if that's important. Thanks, Alexander