make sure the seed is planted a couple inches down (to get a strong taproot) and if its stretching at all you need more light, I would have had this problem but I sprouted outside, weed plants that get proper light are very compact, people see plants that get proper lighting and go woah "that a bushy ass stout plant" when its just some bagseed thats half sativa, plants arent supposed to be super stretched and thin (except with the pure sativa, theyre tall thin buggers) A friend of mine had the same exact problem the seedling would get a few inches then just fall over and die, more light plant the seed just a bit deeper. It may take longer to crest/surface but it will be worth it. my gal about a month I bent over as far as a could (I was told with LST you can bend it as far as it doesnt tear out the soil. if I wanted to I could have laid the stem in the dirt and have it pinned totally horizontally, thats how strong the roots should be in a proper plant one month old.