Lightmapper help please !

I have used the lightmapper successfully with a test level created in Maya and imported into Unity. It works well. However, when I apply lightmapping to a level that I had previously created, rooms appear too dark and lightmapping doesn't seem to be working.

( Settings: A light was marked as bake only and participating meshes were marked as static. UV lightmapping selected on fbx importer and models UV Mapped in Maya.)

Any ideas please ?

Thanks