It takes a very long time for anyone to do the flip.
My DD's experience with the flip change is similar to what you state, but once she starts throwing it consistently, it's a deadly pitch. Many beginner's tend to slow down the arm circle to take off the speed of the pitch. Do not do this! The arm circle should be the same speed as the other pitches, its the lack of arm whip which lessens the speed of the pitch (ideally 20 - 30% less speed than the fastball) that is critical. Everytime she practices pitching, make sure she gets some opportunities to practice the c/u. Also, have her throw it in games, whether or not she has "perfected" it.
I agree, when she releases right, the flip is nasty. Her problem is releasing too soon & too late and she's just way to inconsistent to feel comfortable throwing it outside of practice. She knows it doesn't need to be thrown for a strike, but it can't be 10ft high or rolling on the ground.