Is it loosening itself or is it still breaking in?
As it breaks in it needs to be re tightened as the grooves in the rings form and the balls seat.
*Hold one wheel at a time and run it gently and adjust the tension til it gets the right feel (about 1 full rotation after spinning/flicking one wheel while holding the spur gear) and no longer needs further adjustment.
*Diff break in procedure on page 1 tips section of this thread