How does fatigue affect hearing ability? I can't hear what's going on when I'm tired.

Fatigue and hearing. What you describe is most likely a function of your central hearing mechanism, or processing. I suspect if we looked at your actual hearing levels, they would not change even when you are tired. However, your attention, focus and processing are all impacted by fatigue and are all necessary for auditory (hearing) processing. Obtaining an objective hearing test is always an important first step.
It shouldn't. Perhaps you are not focusing - your objective hearing ability does not change due to fatigue, but your concentration and thus comprehension may decrease.