quadcopter flipping

My quadcopter flipping from tail.
I have already checked directions of our motors and propellers respectively.
I am using ESC of simonk firmware of 30A and we have already calibrated it.
I am using arduino nano with gyro GY88 and Multiwii Code.
My query is to find out the obstacles that why my quadcopter is flipping after all the finals checks ?
Your response will be highly appreciated !

Try resetting the gyro. Are the motors in proper sequence
1=back right
2=front right
3=back left
4=front left
& other than prop direction I don't know as I use SPF3 or Piko.