The widget or notification says "Update Error" or never finishes loading

Power and data saving features on the device are by far the most common reasons for this error.

When the app is not open, the widgets and notification are considered to be running in the background. The system is much more aggressive in restricting the app while it is in this background state, and often results in a failure to complete the update.

* Here is a trick with the notification that can help you figure out if this is the reason. If the notification doesn't update on its own, try opening the app and leave it open. If the notification updates with the app in the foreground, voila! You can experiment with manually updating, with or without the app open, but keep in mind the system may temporarily give the app foreground status after you interact with it. Android has several built in features and phone manufacturers may have their own custom battery saving features.

A common Android specific feature that can prevent updating on some devices is called battery optimization. Disabling optimization has worked for many people.

There are some manufactures that are particular bad about killing apps in the background. A website has been created to draw attention to this problem. You may find some helpful information for your device there.

Notification doesn't show up in the status bar at all

This is usually a permission error instead of a power saving feature. You may have (even accidentally) blocked the app from showing notifications.

You can find the notification settings in your device settings. A shortcut on my phone is to long press on the app, then the info button, and notifications will be listed there and you can check how they are configured.