A lot of people have been telling me that they can’t get into ClockworkMod recovery to save their life, even if they’ve flashed the recovery in fastboot a thousand times. Well, that happened to me after trying to get into CWM after installing the latest ICS ROM.
This is some kind of security added after Android 2.3.6 ROMs that messes up your ability to get into CWM.
If this happens to you, you can still boot into CWM by flashing recovery to boot partition. This will basically boot your phone into CWM every time until you flash a new ROM.