Center of Mass Calculation Problem.

Hi,

I have a problem with center of mass calculation. My bubble is rising in z direction as time passes, and the simulation shows that to me correctly. However when I want to calculate center of mass in z direction from results>derived values>global evaluation, that center of mass does not change with respect to time, it always gives the initial value of the center for all time values.
What could be missing in here? Since I am new to COMSOL, it is complicated for me.