It seems there is a bug with MOUSE_MOVE event in fullscreen and mouseLock activated.
I've explained the behaviour here.
You can use this link to test the problem: in fullscreen with mouseLock active with a constant movement always in the same direction (with a very slow movement is easier to reproduce), the player show "zero values" for movementX and movementY every two events fired.
My flash player version is 11,7,700,203. The behaviour looks like a bug: if the current mouse movement (movementX and movementY) is equals to the previous movement, than flash return movementX=0 and movementY=0. The next mouse movement will be the correct value because it will be different from zero.
You should move the mouse in a direction (only in x or only in y) very very slowly (to move only one unit at time) to maintain the constant speed to reproduce the bug, otherwise you should make micro movements in a direction.
I hope my English is understandable.