When a power source is placed adjacent to a slab, the source doesn't power redstone dust traveling down the slabs. See the attached pictures.

In the red example, the redstone dust fails to power the redstone lamp, when it clearly should.

In the yellow example, the power source is placed on the bottom of the slabs, and the redstone lamp was placed on the top of the slabs, but it succeeds in powering the redstone lamp.

Reloading the world, restarting the game, and restarting the console doesn't appear to fix the issue.