Exposure adjustments not working on iPhone pics?

Pulled a collection of iPhone 6 photos into Aperture to color grade and correct. The 'Adjustments' seem to work on 'Highlights and Shadows', but not on 'Exposure'. The Exposure controls seem to have no effect on the image.

Wondering if I have to convert the iPhone 6 formatted photos to some other format before being able to 'adjust' with the 'exposure' controls?

Or what else might be the problem?

All ears,

Ben