System is good, but continually fall from the cameras

I already have a camera returned (Complaint at Amazon 1A worked); the camera then works (more coverage, the first has not even sent 10m without obstacles), after 2 months it falls and off again, although the locations of the stations has not changed. I have a 2 camera system, the second shows the same behavior. Super is the app at the I-Phone, you can always look up (as long as the cameras just work), what is going on at home.