Extruder motor doesn't turn

Hi everyone,

I am building a I3 Rework and I am having an issue with the extruder stepper motor.

Before mounting all the electronics on the printer, I decided to test it. I am using Repetier-Host and Repetier firmware.

With the manual control of the repetier-host, I am not able to make the extruder motor turn (Neither to extrude nor to retract) whereas the other motors can.

I exchanged the extruder motor with the X-motor, and I had the same problem : so the motor is not the root of my problem.
I exchanged the Pololu A4988 from the extruder to the X-axis, and I had the same problem : so the A4988 is not the root of the problem.

After some searches on the web, I found that it can be due to the temperature of the extruder (In fact I plugged only the thermistor that said 20°C).
So I heated the thermistor with the resistor in the hot-end up to 175°C... I was still unable to make it turn with manual controls!

Is it a common issue ?

Secondly, the two motors of the Z-axis are turning quite slowly (compared to the X and Y motors) and it seems that it is intermittently. Any ideas of what can be the problem?

Thanks in advance :)