I spent a whole day trying to figure out why my foliage weren’t making any lightmaps, everything else in the scene had a lightmap but when I switched to ‘lightmap density mode’ I could see that the foliage didn’t have any lightmap at all (that strange beige color). The message logs gave me no errors (just a warning about the LOD’s which should be irrelevant).
Eventually I figured out that the solution was to restart the engine. That fixed it!
Nowadays I’m finding myself having to restart the engine around every 3rd-5th lightmap build. Sometimes this is no big deal, other times this is super annoying when I’ve had it render all night long for a deadline only to discover in the morning that I had forgotten to restart the engine before building my lightmap.
It’s not caused by lightmass being out of memory, as my ‘windows task manager’ shows I’m never even using half of my RAM.