I have Lightmass importance volume and Lightmass portals:
So, in the first image corners seems have some AO.
The second image have just bad shadows and i can’t identify the problem.
In the third image shadows are imprecise.
Did you set the lightmap resolution for the mesh, and exported the mesh with a second UV maps? In rendering tab has a field to set the lightmap resolution.
You need to split it up into more pieces, if it has a lot of surface area then you can’t get as good of a result with a high resolution lightmap. Also make sure that no meshes are extending beyond each other. Beyond that you can try increasing the lightmass quality.