Why are push notifications not being received by a specific User ID, even though the system indicates they were sent?

If push notifications sent to a specific User ID aren't arriving on the intended device(s), the cause is almost always the device record itself rather than a registration limit.

About the device limit: a single User ID can be linked to up to 20 devices. This limit does not block new registrations — when a 21st device is registered, the oldest inactive device is removed automatically. So a tester who reinstalls the app on many devices may find an older device silently unlinked, but the newest one is always registered.

Here's how to troubleshoot:

1. Check the devices behind the User ID: go to Audience → User Explorer, search by User ID and open the Devices and platforms tab. Confirm the target device is listed and that it has a valid push token for the expected platform.
2. If the device is missing or has no token: it was either evicted as the oldest inactive device, or it never completed registration. Reinstall the app on that device, allow notifications, and perform the action in your app that assigns the User ID.
3. Clean up stale records (optional): old device records can be removed with the /deleteDevice API method by HWID. This is housekeeping only — it is not required to let a new device register.
4. Verify and test: after re-registration, confirm the HWID is linked to the User ID in User Explorer, then send a test push to that User ID or HWID.
5. Check the delivery report for the message (Received Messages tab, or the /getMessageLog API) to see whether the push was actually handed to APNs/FCM for that device.

Important note: Pushwoosh delivers notifications to the platform gateway (APNs for iOS, FCM for Android). Final delivery still depends on the gateway and on the device's state — network connectivity, battery optimisation, and app notification permissions.