I think this is because cleavers were recently altered to require slightly less fatigue for uses. Or it might be to do with the reduced weapon fatigue costs that come with weapon mastery. If you have Cleaver Mastery and are exactly 9 points below your max fatigue level, you can’t use cleavers for an attack, even though the skill is highlighted as being available. Instead, when you click to select the skill, the Max Fatigue bar starts to flash to indicate that not enough is available for the skill (not captured in the screenshot).