Make sure that you have allowed location services in your phone settings.
Ensure that the app is running in the foreground or background and that you are logged in. If you are logged out or the the app is closed, the app cannot send your location, and you will not be automatically checked in.
Your phone will need to have a good GPS signal and the app will need to be running while you are driving. Here is an article on GPS and Manual Locations
If you have an iPhone running iOS12 make sure to check out the iOS12 article.