Is it getting enough fresh air? Some of my Neps had problems with rotting tips when I had them in a tightly sealed terrarium without much air movement. What kind of conditions are you providing?
On a tangental note, when asking "Why is my plant doing <blank>?"-type questions, it's important to include lots of detail. You're not just asking a question about your plant - you're asking a question about how you grow your plant. So, if someone is to give you a useful, specific answer, they need to know about the whole setup. Your plant is only a portion of the equation, so to speak; the plant doesn't grow all by itself. It needs light, air and water to derive energy, and nutrients and water to grow, good media to protect the roots, etc., and all of these things influence growth. Without this information, people can tell you lots of things, but all of the advice you get will be based on people guessing about what might be wrong, rather than considering your situation in particular.
I think that asking good questions is a very important general skill, one of the most neglected parts of common sense. I don't mean to rant or lecture about this - I only mention it because you're still in school and at this point these kinds of skills can literally change your life.
Ask good questions, clean up after yourself, and always read the fine print - three things you can never go wrong with.
~Joe