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 location faster)
More information
If you still need help, please try solutions from Google:
https://support.google.com/maps/answer/2839911?co=GENIE.Platform%3DAndroid&hl=en