Do a quick check if GPS settings have been enabled and permissions granted? You will also need to enable Background App Refresh.
Go to Settings -> General -> Background App Refresh and turn on this setting.
Make sure that it is enabled for Detrack in particular. Background app refresh allows more than one app to run so in the case of navigation, you can use the Maps function and at the same time Detrack will be able to feed location back to our servers.
After the checks are done and GPS is still not working, try rebooting the phone. Sometimes, Android geocoding service will crash and a reboot will fix the problem.
If after performing the above and GPS is still not working for you, please open a ticket and let us know the model and make as well as the Detrack ID of the device, this will help us to speed up troubleshooting.