Lighting build in persistent level hangs at 99%

I am using a persistent level, with has 3 sub-levels that I initially had separated. Since switching to persistent, whenever I build lighting it loads to 99% quite quickly then hangs there for much longer than it took to get there. When it finally finishes there are usually errors with the lighting. The largest of these sub levels is only ~300 actors and the average lightmap resolution is 128.

Thank you for your help.

are you using lightmass importance volumes? if so try to use one for each section independently, and size it accordingly. This may not solve your issue, but never hurts to ask.