Help Please - One of Z axis Motor CANT CONTROL!

Hi I have Mendelmax 1.5 with Megatronics v2.0 running with Marlin firmware.
Everything is fine except one of the z axis motor. When I run printrun to test homing, axises I see that one of the z axis motor is not working good enough. Sometimes it cant rotate. Everything (include the config.h, motor itself, assembly, wiring, accelerate the current of the motor) is fine. But when I play with the current of the z axis on the megatronics the motors movements are changing. Sometimes it cant rotate. I know turning CC will accelerate the current and CCW wil decrease the current. I'm focussed on the current now. The motor is new and electrical stuff - wiring is ok. Any ideas ? What can cause that one of the z axis motor is not running good enough or try to rotate but cant. And the part on the motor's head is the same as the working motor- as I said the assembly is ok-. I'm gonna lose my mind if I cant find what problem is. PLEASE HELP ME. I will send you all the details you need. THANKS.

1)These are not my config but I have no idea what value must be done for my printer. If you can help me that maybe this can cause proble as I search on this forum.

#define DEFAULT_AXIS_STEPS_PER_UNIT {15.6302,15.6302,760,760*1.1}
#define DEFAULT_MAX_FEEDRATE {500, 500, 1, 25}
#define DEFAULT_MAX_ACCELERATION {9000,9000,100,10000}

My config.h file on pastebin:
[pastebin.com]

2) I havent measured the vref and the current on the motor driver or motor if someone help me how to measure them maybe that can be my problem too. I can re set the current of the z axis.
I have StepStick Driver: STEPSTICK8825 - you can check it from here:
[reprapworld.com]

Here is the helpful video of the my printer. This video can help you to solve problem easily:
[www.youtube.com]

One of the z motors is skipping steps.

Can you turn the threaded rod by hand with the +12V power off? Maybe it's binding
Are the z motors identical to each other?

They don't have any instructions for measuring Vref in the datasheet, but you can just try increasing Vref a bit by turning the trimpot on the z controller a bit clockwise.

Edited 1 time(s). Last edit at 01/30/2014 10:11AM by brnrd.

Thanks for the reply brnrd.
I can rotate the rod and the motor when the power is off. Motor is new. And wiring is ok. I just don know what is the perfect current. I'm accelerating the current but curious about the motor. So ı cant accelerate more. Maybe that can solve my problem but I think I have accelrated much enough. Is there something else I can try?

do you have the motors in series or parallel?

Hey guys I have a question:
I have wired only one motor on z axis and change the step stick driver on z axis and the motor is new and cables are ok.
When I press home Z, the motor makes sound a little bit and before finish homing motor rotates finally BUT AT THE BEGINNING JUST MAKES SOUND. MOTOR ROTATES AT LAST A FEW SECONDS BUT DOESNT ROTATE FROM BEGINNING? What is that