I have had a snake plant (and its multiple progeny) for 35 years. For the past several years, new leaves come in look fine and then end up with a narrow, almost round stalk, with only the top couple of inches flat and wider. Even that part is much narrower than the leaves used to be. What is wrong and how do I get it back to having the uniformly broad flat leaves?
This could be watering issues; allow soil to dry between waterings.
Your plant may also not be receiving enough sunlight. A bright east or west window location would be best.