Light Building Errors in the scence.

Hi, I created this scene to practice lighting. I have already entered the final step and it is rendered at production quality. The outcome seems to be ok, but I found there are some errors and I want to know what contributed to them. Please take a look at the detailed lighting image I uploaded. It was rendered with a very high lightmap density, but you can find in the two highlighted areas the shadows seem to be very strange.

At the very first I believed it was the lightmap that caused the issue. However, from the screenshots, both the lightmaps on A and B are evenly applied. Besides, I found both A and B are relatively small and complex so I was wondering if it could be caused by the shape and size of the static mesh but I found the kickboard, which is also complex and small in the scene, is free from the issue. I am totally confused, can anyone tell me how to fix it?

For your information, I used two types of light, with directional light to mimic sunlight and spotlights plus bounce boards to perform as the skylight. You can see the light arrangement from the final image I uploaded.

Thank you very much.