Maybe the appointment time already passed?
Maybe the villager wanted to cancel and never told you?
It has happened to me that a villager was like "Oh, I totally forgot to do X with so and so.. sry can't make it" without ping :T
it's possible that they cancelled or totally forgot to show up. i've had villagers forget to show up and they never mention it again. i oftentimes send them a letter saying that i'm pissed lol.