Screen is too big for the lights; is allowing a thin growth over a large area, which is getting burned in the middle because thin growth is not thick enough to absorb the strong light. Needs instead a small growth area right in front of the light, about 5" high, which will concentrate growth into a thick green pile. If you can't rebuild the box, then try adding iron, and reducing hours. You could also try putting both lights on one side, to try to make use of the huge area. Lastly, you could try doubling feeding.