If you've ever tried to share something with a friend or family member over AirDrop, you might have found that their device doesn't show up in the sharing menu when it should. While there might be some kind of software bug at play, the more likely culprit is the Do Not Disturb mode on iOS. If the device you're trying to send to has Do Not Disturb turned on, then it will not appear in the AirDrop section of the iOS share sheet.
While putting a device into Do Not Disturb mode has prevented people from AirDropping things to it for some time, this piece of information has not been shared widely. Developer Guilherme Rambo noted this feature when trying to find answers for himself.
It should be noted that Do Not Disturb only hides an iPhone or iPad from AirDrop if that device is using a different iCloud account from the one that is sending something (or no iCloud account at all). If you're sending something between two devices using the same iCloud account, having one of your iPhones or iPads in Do Not Disturb does not keep it from showing up in the share menu of other devices connected to that account.