Z axis lose steps

Hello everybody, a little thing with my z axis is that "losing steps". Axis moves pretty well by hand and when I give him order to move 10mm it moved 10mm but all my prints at height is smaller. I print a part 10mm and is 7.6mm, I print a part 96mm and is 62mm.

I use 2 nema 17 and connected by series, I reduse maxfeed and acceleration and steps I let at 4000.

Do you have any suggestion? Should I try connect them in parallel?

Thanks Panos

Diagram for wiring
It appears they are wired in parallel in this picture, improperly tuned steppers and overheating motors or drivers can also cause steps to skip.

---

Prusa i3 Rework - Ramps 1.4 - E3d Lite6 - Full Graphic LCD Controller

Most Nema 17 motors used in RepRaps are better connected in series, but low current high inductance ones are better connected in parallel. So we need the specification of your motors to be sure.

As well as reducing Z feed rate and acceleration, you may need to reduce the Z jerk speed.

---

Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Are you sure that 4000 is the right steps/mm? What threaded rod do you have driving the Z axis?