Sometimes I don't understand sarcasm?

What can be the reason if I sometimes don't understand sarcasm? There's a girl at work. I never understand her, she looks and talks seriously all the time. Also, a guy too, at work.

But other times, I don't have this problem with other people.

Could the reason be that I don't actually pay attention to her or might I be autistic to a degree?

Sometimes I don't understand sarcasm?
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