Turns

Hi, I'm having trouble with the freestyle tumble turn. My freestyle is okay - not too slow but not amazing. However, I am absolutely horrible at the turns. I can't seem to get enough momentum for the turns. I can't turn fast enough, and thus the turn is very sloppy, always ending up with me somehow staring at my feet when my body's underwater after the turn and my upper body too low in the water. I've watched some really good turners turn with absolute ease - mind you, I can swim faster than some of them, and some of them just seem to glide into their turns with no speed at all, but they build up huge momentum during the turn, finishing the turn with their legs almost straight and very far away from the wall, but still be able to get a good pushoff that's not deep at all. So, how do I get my tumble turn to work? How can I turn fast enough and with my legs relatively straight? When I turn by umping off the floor of the shallow end I turn like the good turners, but when I swim into the wall I can't turn anymore, which leads me to believe that I'm not getting enough momentum.