OK well I think the cord was stuck in the grommet and that's why I couldn't move it last night.

Anyway, I've decided NOT to move the bulb down. When I put my head at the tops of the plants in the corners, I can still see the entire bulb when I look up at it. Therefore, lowering the bulb won't do much. But I think they're getting enough light I guess. If I can see the bulb from the same vantage point, then the plant can "see" the bulb too.