Camera not reporting motion for specific person.

Strange one this. Our stick up cam with battery is working really well for our elderly relative where we're covering the door entrance. Except, the last couple of visits by the carer have either not registered or the recording is blank. Is there a way the camera can be interfered with so that recordings are not made? I can understand she doesn't want to be recorded but it's only for the moment where a person is coming in or out. Our priority is watching out for our relative with dementia wandering out at night time, although knowing that carers are on time is a reassurance. Any thoughts please?