First check that you've given the app access to your GPS location; without that access, check-in doesn't work. Also check your internet connection, as the app needs a connection to check in. Do you see a notification but no button? Try via Bookings > Check in.
Still not working? Take a screenshot of what you see and contact us. We'll investigate what's going wrong and, in that case, approve your missed check-in so you don't get a no-show fee. Read more about checking in at workit.nl/gps-check-in.