I updated the iOS app on Monday (4/6) on the three iPhones of my family (6, 6+ and 4s), and I’m having major problems with all of them. They work when I set them up, but they seem to lose background location notifications “after a while”. The only way to wake them up again is to open the app which is running in the background.
When I had similar problems in the past (I’ve been using iphones as presence sensor for >1yr now), I went in the iOS settings, and disabled/re-enabled the location permissions, background running and cellphone data usage setting for the ST app; that always fixed the problem. I’ve now done the same a few times, but the problem keeps coming back, and on all phones.