Question Milling Motor MM-1000 estos in portal mode

Hi, I'm Óscar from Spain and I have an d840 for about two montes.
From the purchase an assembly of the machine and adjust it I only can machine a few works because I suffer two main problems:
- First the z axis stucks at lower position, at time to go up the nema 17 doesn't hace enough power to do that. I trying change the wires up to down and viceversa and check that the problem its only at up movement. I change the motor and seems that have a little bit of strong to go up, but I still stopped only pressing with my finger. Main board? Motor? Any idea?
- Second when the milling motor its connected in portal mode stops the milling procces at the middle of the work, if I disconnect it from main board and use in manual mode finish the work without problems. Help please!!!
Thanks, danke to everyone!!!