Also, keep all your plants at the same level. If you have some plants that are shorter than others, then place them on books or bricks, or something like that in order to raise the shorter plants to the same level (height) as the taller ones. Once you've done that the light will be at the same distance from the tops of all your plants.
A plant, or especially a seedling that is too far from the light source will stretch in an effort to reach the more intense light that it needs. As the others said, keep the light as close to the top of the plants in order to get good penetration of light down to the lower branches of the plants.
At distances of more than one foot from the light the intensity of the light diminishes exponentially (There's a graph somewhere on this site that illustrates this.) So, the farther away the light is from the plants the more they will stretch and the smaller and less dense the buds will be. Without enough light your plants can be sparsely populated with wimpy buds.