Because of the way that your bulbs are positioned, there is not much light shining down on the plants nor is there much light shining up toward the hood (so it can get reflected down on the plant). In fact, the majority of your light is going straight to the wall.
There are two ways of solving this:
You can turn your bulbs horizontally, causing them to shine lots of light up (toward the hood) and down (toward the plant), and not shine so much light toward the walls.
- or -
You can modify your hood in such a way that the light is reflected down toward the plants before the light reaches the wall. (having the hood on top doesn't do as much as having an angled reflective surface between the light and the wall.
Just remember that the most light exists in areas perpendicular to your bulbs, so right now, the majority of the light in your box is going straight to the walls...you can either deflect it down before it gets to the wall, or you can adjust the orientation of the bulb entirely.