I’ve had this too for some models. It might happen for models that have a lot of smal meshes (I had a palm-tree). I could solve it by increasing the “Min Lightmap Resolution” in the LOD0 settings and then pressing Apply Changes. I increased it with factors of 2x at a time. Resolution 128 was enough. I once tried 1024, but then UE hung at 0%. For another model the Light-Build-phase set that even the generated UV’s were overlapping; increasing those also to 128 solved the problem. This is in UE 4.11. Hope this helps.
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