Few things may cause this issue.
The first thing that I usually do are clean cache & validate - look for Cache at the top shelf on the Swarm Agent window, you will see the 2 options there.
Second, if you have a huge mesh that may cause a problem when baking (when I say huge, I mean really huge in dimension)
Third, make sure you use Lightmass Importance Volume
I hope that help.
Cheers