To teach (student x) or not to teach! (Student y) that is the question?

Oh dear, oh dear... have you ever faced this conundrum? I have limited spaces available ever since I decided to cut down on evening teaching (to be a good mother, acceptable wife ..etc..etc).  I get heaps of lesson requests and I don't know what to do? Do I get rid of a mediocre student in the hopes of filling their slot with a potential amazingly talented unknown??? Or do I show loyalty and professionalism to all my current students and keep trying... trying.. trying in the hopes they will suddenly become amazing! It's a big risk. I don't want to swap mediocre for over hyped up (by their mother usually!) Unknown????? Please help!!!!