One of the culprits could be Apple’s “Attention Aware Features," which, when switched on, allow the iPhone to lower sound alerts. If you’ve ever set an alarm on your iPhone only for it never to go off ... Attention is the concentration of awareness directed at some task or phenomenon while mostly excluding others.

Understanding the Context

[1] Across disciplines, the nature of this directedness is conceptualized in different ways. Attention can help us focus our awareness on a particular aspect of our environment, important decisions, or the thoughts in our head.