Try to keep GPS on that helps sometimes, especially with Marshmallow that has been helping a lot. I had the similar issue, now I keep gps ON all the time and bounty notifications are working well for me.
For Marshmallow..I tried this..see if it works for you
1.Please enable GPS and then enter bounty rewards zone.
2.. Whenever you are inside the place, open the app and wait for auto trigger.
3. If you did not get it, then click the bell on the top right (manual bell)