Personalised notifications

Subscribers get three choices behind the bell: All, Personalised and None. Personalised applies automatically when someone subscribes, and it hands the decision to YouTube, which weighs the viewer's watch history and how they have responded to notifications before. A subscriber on Personalised may be told about some uploads and not others.

This explains the most common complaint on the subject. Subscribers who say they never hear about new uploads are usually sitting on the default, or have muted YouTube at the phone level, or unsubscribed at some point. Asking people to hit the bell is asking them to change a setting they may not know exists, and nothing on the creator's side overrides it.