marlim problem

[spa_sergio]

I have a problem with my acrylic Prusa pro b
I've printed several pieces . and when I measure the size of the pieces do not match the size they should have .
I proceeded to calibrate and change the values ​​in the marlin, and continues to do the same. ( repeatedly)
change the values ​​from the screen of the Prussian and no improvement .
the real problem comes with the motor steps that I will not come casket up.
that is to say. I motioned 80.11 for example, and the printer does not change , are always to 78.74 . do so with the marlin or directly from the printer screen .
What could be the problem? a greeting