the bulb manufacturer in my case (sylvania) states that the minimum distance from the plants is 45cm=18 inches.....try to lift the lights up a bit and see if it works....also the 250watt that is up and running for 18 months in a row, is no good either.....you should change the bulb every six months of continuous operation...even though the manufacturers state the average life of the bulb is longer than that, after six months the bulbs loose in efficiency but they still consume the same wattage anyway!!!!