Lightmap density issue on Floor blueprint

Hey Guys, I`m new here. There is some issue with lightmap density on the floor since I made it inside blueprint for realtime changeble materials. On the screenshot you can see the lightmap density is shows purple and it looks like dont have enoght sqares to catch lighting. So after baking lights there is so poor quality, cant see anything. Only the sun which is moveble. How could fix it inside bueprint?