Why aren't my rockets working?

Hello, I am 23 and I have just gotten into the rocket hobby, and I have started making my own engines. I am primarily using a NH4ClO4+Al+Epoxy fuel of my own proportion, which is very powerful. My problem is that the tops of all my engines are blowing out and, therefore, not flying. I don't know why none of them are working. Does anyone have any suggestions as to how I could make them better? If so thank you.