Why don't existing members see default channels I added to Discord Onboarding?

That's expected with Discord Onboarding: Default Channels are the channels new members see when they join, and adding one later doesn't reliably put it into every existing member's customized list. Have current members open Channels & Roles and use Browse Channels to add it, or put the channel behind a customization question that existing members can answer there. Also check the channel's permissions, since Onboarding doesn't override them. For anything everyone truly needs to notice, it's worth posting an announcement that tells existing members exactly where to add the channel.