A common bug associated with Uber is related to the GPS locator. Often the app was locating customers incorrectly and thus not sending cars to the right address.
To avoid bugs, the most important thing to check is that you have downloaded the latest software updates. As soon as bugs are reported, developers are pretty quick at getting a fix out, but you'll only get it by updating the app through your app store.
Why does the Uber app not work?
Usually if apps are not functioning properly it is down to bugs. Because developers are constantly improving and adding features -and trying to roll them out as quickly as possible- there can be aspects that don't always work perfectly straight away.
If this doesn't help, then you could try completely uninstalling Uber and downloading again. You might have to re-enter some of your personal details and preferences, but often a fresh install can help if you're having problems when everything seems like it should be working.
If all else fails, you can try contacting their customer service department, "Rider Support".
They have a pretty comprehensive help section, and you can submit requests for assistance via the website, and send them screenshots of any issues you're having.