View the original community article here
Last tested: Aug 28, 2017
A possible reason for this is that the Custom Map Layer property key names have to match the corresponding Dimension string fields.
|"dma1":"New York, NY"||New York NY|
Where "dma1" is the Property Key that is mapped to the custom map layer definition. The Property Key is configured correctly, but if the respective strings do not match then the map will show blank.
User either needs to add a " , " to the LookML Dimension values, or remove the " , " from the TopoJSON "dma1" values.