Can't see the light(s)~!

[orwake]

I have some light fixtures that I can't get to behave properly. They were working fine in the same file and then, poof!, now they don't. I have checked all the light group settings to be turned on while rendering (raytrace) but they don't provide any light for my scene. First it was just recessed downlights in a sloped ceiling, but now, the fan won't work either. Wall sconces are still on or I would be in total darkness. Can you help?

[gravelin]

Have you check up the Accurender matérials you used ?
They MUST be transparent .

[orwake]

Thanks for your response, but I can't find a parameter choice for materials for the recessed downlights that come with the Revit 8 Library so how could I change these settings? I loaded the same lights in a different project and they worked. Also, in the "scene lighting" table, my ceiling fan is checked on but greyed out. . .what does this mean and how can I clear it? Did I mention that there is a sloped ceiling in this project? Could that have anything to do with my problems? (I did check a section view to make sure the lights are at the proper level and not trying to penetrate an opaque surface)