Like FMRAM pointed out, schools can be hard to get. Actually, schools ARE hard to get even when you need the school for your rate or even for promotion. It's just the way it is. However, this doesn't mean you need to accept this "as is" for your case. In my experience, I have found that very, very few people know how the system works in order to get schools, even the training petty officers who are responsible for getting the schools you want/need. What I always did was make sure I knew more than they did in scheduling schools for my guys. Sure, the "official" procedure is to go through the training bubbas at your command, but I always found that method to be inefficient and not quite as good at getting the desired results.
Your first battle will be TAD money. If the desired school you want will be "no cost TAD" then you have a much better chance of getting it. If however, your school does require TAD funds, you better be prepared to make the case for why the school is necessary for you and how it will help the command. If you can't do either, forget it.