Amazon FBA not fulfilling orders?

I have orders from 4 days ago that Amazon still hasn’t fulfilled yet via FBA. Is this normal that sometimes Amazon just doesn’t fulfill your orders within 24 or 48 hours? Some of these orders are business customers if that makes a difference.

Also I’ve gotten a few returns because of “shipping address unavailable”. Should I reach out to those people and see what the issue was? I’m worried they’re going to have a bad experience because of issues with Amazon’s fulfillment.