In short: it's usually one of six things. The ring/silent switch, an active Focus mode, the per-app sound setting, ringer and alert volume, a notification permission that got revoked, or, if the schedule is wide, iOS's limit of 64 pending notifications per app.
The first two catch most cases. The rest take a minute each to rule out.
The small switch on the side of the iPhone silences every alert sound system-wide, including notification sounds from every app. It is the single most common reason a chime that worked yesterday is silent today: it gets flipped by accident in a pocket or a bag more often than anyone notices. Check it first, before anything inside Settings.
Focus (Do Not Disturb, Sleep, Work, or any custom Focus) silences notifications for apps that are not explicitly allowed through it. Two things trip people up here. First, a Focus mode can be scheduled to turn on automatically, so it can be active without anyone having switched anything by hand. Second, and more important: chimes missed while a Focus mode was on are not queued up and replayed once it ends. They simply did not sound, and there is nothing to "catch up" on afterward.
To fix it: open Settings, go to Focus, choose the active Focus mode, and add the chime app to its allowed apps or allowed notifications list.
Each app has its own notification settings, separate from the system-wide ones. In Settings, go to Notifications, choose the app, and check that Sounds is turned on. This is easy to miss because it is independent of whether banners or badges are enabled: an app can show a banner or update its badge with no sound at all, which looks like "half of it is broken" rather than a simple switch being off.
The volume for ringtones and alerts is a separate setting from the volume for music and video, which is why a chime can be silent even with media volume turned all the way up. It is set from the Sounds & Haptics section in Settings, or with the physical volume buttons while nothing else is playing and the screen is on the home screen or lock screen. If it reads as very low or muted there, notifications will be too quiet to notice even though nothing else is wrong.
Permission can be turned off after it was granted, whether by accident, during a "review app permissions" pass, or from an iOS prompt that got dismissed without reading it closely. Without it, an app cannot deliver any local notification at all, sound or otherwise, and there is no partial state: it is either allowed or not. Check Settings, then Notifications, then the app, and confirm Allow Notifications is on. A well-behaved chime app should say so on its own main screen when permission is missing, rather than leaving a toggle that quietly does nothing.
iOS caps every app at 64 pending notification requests at a time. This is a system limit with no setting to raise it, documented in Apple's own UserNotifications reference. It rarely matters for a plain hourly chime, but the moment a schedule adds finer marks, the math changes fast: every hour of every day at the quarter, half and three-quarter mark works out to 24 × 7 × 4, which is 672 separate triggers, ten times over the limit.
When an app hits this ceiling, something has to give, and different apps handle it differently. A poorly built one just stops registering the newest requests, so entire days or the finest marks go silent for no visible reason. A well-built one spends the 64 slots on coverage instead: the top of each selected hour first, then finer marks, rotating through days so none of them goes completely quiet. Either way, the fix from your side is the same: narrow the hours, the days, or the interval, and reopen the app so it can rebuild the schedule inside the new, smaller total.
The one-minute check, in order:
Tinny(not) shows a warning on its own main screen the moment notification permission is missing, and spends its 64 slots on coverage rather than letting whole days go quiet.
See Tinny(not) on the App Store