Not sure if you've got a defect in your particular lens, Ian, but generally, for the PDAF system of the OVF, there has to be a line to focus on. Not an area, not an object, a *line*.
For the CDAF system of Live View, the difference between light and shade under a focus point gets (eventually) maximized. There is a special exception for Face Recognition, if you have that turned on, it looks for two eyes, AFAIK.
A featureless surface like a painted wall or a clear sky can present problems to either method, racking being a symptom.
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