Both Apple and Google are trying to restrict background processes to give user better battery life, but it affects application like Traccar Client which require persistent background execution. Common issues to check for:
On iOS if you swipe app off the screen, operating system actually kills the process and app will no longer be able to report.
Make sure that in iOS settings background execution is allowed and Traccar Client has permission to always access location services.
On recent versions of Android make sure that you have added Traccar Client to battery optimization exceptions.
Some Android vendors have their own battery optimization in addition to standard Android system. Make sure those are disabled or the app is added to exceptions.