We are using some older rockets as an example for the motor mount and fin design.  The two red rockets are from 2 and 3 years ago.  Notice the placement of the centering ring and the how the motor mount is positioned.  On the right are two rockets from this year.

Old Rockets                                                                             New Rockets
  

There is a position relationship between the fins, the fin tabs, the motor mount, the fin slots and the centering rings.  They all need to come together in a dry-fit, before the big glue-up. You also need to attach the launch rail button before you try to glue everything together.