Hi Baykush,
first of all please tell us what kind of cetus you have. the small or the big one. Then it would be helpful if you provide fotos, or a video (youtube) here. also the value you put in the cetus application for Z-Axis is quite important.
If I remember mine is around 180mm ± the distance to the 9points on the plate.
In case you did everything right it could be an issue with the z-Axis motor, or the belt. As the cetus is only using the motorsteps (or revolutions of the stepper motors) for measuring the distance it seems that there is a problem. Sorry but out of your one sentence it´s hard to solve your issues or help with advice.
best
kirkdis