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