The GPS may be sometimes imprecise if you're inside a building. Therefore, if your phone's GPS cannot collect the location and you are unable to start or finish your shift, here's what you should do:
Go to the Settings of your phone
Find Location settings
Go to the Mode and select 'High accuracy'
In addition, try using WiFi (turning WIFI ON makes GPS find a location faster)
If you still need help, please try solutions from Google. You can contact them here.