Add button in Override Materials property is not responding in Skeletal Mesh > Rendering section in detail panel. This is not responding from 4.15 and above.
1) Add Skeletal mesh component into any blueprint.
2) Search for 'override materials' in the detail panel.
3) Try press + button in the Override Materials property to add a new element.
Notice that from 4.15 version and above this button do not add a new element.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-45768 in the post.