I was having the same problem and a coach said that it is more balance than propulsion. I watched a youtube of Shinji doing Superman glide and he keeps balanced even when he is only moving centimeters at a time.
Mat Hudson said try and keep your body long and stiff like a board. This is turning on some balance muscles. Once you stay more in balance you will train those muscles.
My own personal cheat on this was to make sure and take a full breathe. It keeps me on top better and more balanced.
Once you get the hang of it try and be more relaxed because tense muscles are not good in the long run.