z axis not being able to go down

[amodelanoche 0]

I have a prusa i3 pro b mk8 and I'm using cura.

 

The z axis doesn't go down. When I make it move up it resets he z to 0 on that height (sometimes it doesn't set it back to 0 so it can go up and dow until that spot where it last did a z reset).

 I've already checked the wiring, and I also checked that the motors work. Sometimes when I try to make it go down via cura, the motor makes a weird sound but it doesn't go down