watchOS 2: setting properties on initial Interface Controller
You might move your code to applicationDidBecomeActive.
This page describes the states of watch apps. When applicationDidFinishLaunching is invoked, the app is in an inactive state.
If you are calling this from within another interface controller, try move the WKExtension.sharedExtension().rootInterfaceController to the willActivate() function. It seems like if it is in the awake() function it sometimes works but is unreliable.