It's been a while, so I hope you already got past this, but if not...
If only the Geofence Monitor is running, it can take a while for the location to update, so it may actually be working but you're just not waiting long enough. What I've done is to keep the Location Monitor also running, on low power, updating at 30 second intervals. The Location Monitor will stop by itself (I have no idea why, that's just the way it works), so I've set up a profile that runs every hour to turn it back on. That should cause the geofence to update at 30 second intervals as well.