Flaps can be extremely challenging!
There are students everywhere who struggle for a long time before being able to do flaps correctly. In the course Technique 3 – The Flap, there are 16 videos where we practice the feeling of flap, the position of the leg, the movement, the relaxation in the ankle. All of this is to lead us into using the correct technique. Once we are flapping, we try moving in different directions and we do some triplets and syncopations, too. There is even an entire flap Exercise Choreography (mp3 included) to keep on practicing. This is a huge amount of material to help perfect flaps!
The key thing to keep in the back of your mind here is that if you are struggling with doing flaps or teaching flaps, consider not worrying about the sounds as much as searching for the correct feeling and the correct movement. Then, over time, everything will eventually fall into place!
Good luck, everyone, and let me know how it is going!