Fatigue can be insidious like being hypoxic or drunk. Some folks I have flown with have been fatigued and a rested crew member can pick up on it. If all fatigued, they will press because everyone is too fatigued to realize, just like the one too many drinks and one doesn't realize their impairment.
We all manage many distractions (wx, schedule changes,etc) on duty and get assimiliated to distractions off duty (sick kids, being awakened on layover without REM sleep, poor diet, etc) that can lead to fatigue without being aware.