why won't my apples redden?

The tree is about 3 years old and 2.4m high. It only has a width of about 800mm as it's a column variety for small areas.
It does abundant fruit. The biggest apple in the photo is about 75mm diameter and has been on the tree for 2 months. I've tasted some. They are 'white/opaque' inside, taste fine, and haven't given me a stomach ache. More of a soft bite than a crisp snap.

But why won't they redden?