What exactly do you mean by a loose ankle? Do you mean you have trouble controlling the orientation of the ankle, or that it tends to fall to one side when you put weight on it, or something else?
I will say that an extremely off-center blade affected my ability to spin, and that I eventually decided that reshaping the interior of the boot - mostly by making my own custom-cut insole, proved more effective for me. But I don't have either of the problems I just named - there was just a mismatch between the shape and tilt of the bottom of my foot, and the footbed of the boot.
If you have trouble controlling the orientation of the ankle, I think a stiffer boot and a better fit might help. What type of boots are you using? How loose do they feel? (E.g., can you slide around inside the boot, or touch one part without touching others?) Were they heat molded?
Also, when you are balanced over one blade, is the blade straight up and down, or does it tilt one way or the other?
Sorry for answering a question with many questions, but I think anyone would have trouble answering you without more info.