Hi Fred -
First I would recommend that you take a look at the Swarm Troubleshooting Guide on our Wiki:
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I would also make sure that you have a Lightmass Importance Volume in your level. If that does not work, try adding in the SM_Template_Map_Floor from teh Engine content to your level and rebuild lighting. This was a bug reported in 4.9 which caused Swarm to be unable to complete the Light Build and is corrected in 4.10.
Thank You
Eric Ketchum