Just a wild guess, and please let me know if I got it right. When you do one on the floor beam, you change it from the way you do it on the floor. I think that's a normal reaction to transitioning from the floor line to the floor beam, but it only slows down your progress.
Do them exactly the same as you do them on floor. Use the same hurdle speed and punch posture that caught your coaches eye in the first place, and follow through with the same body positions you know have worked in the past on floor.
Do them like dunno says, and if possible do them off the end of an un-padded low beam onto that slightly elevate surface (just don't fall backwards) and add to it's height every few days until you know you have the skill figured out.
