Help with geeetech prusa i3 z-axis stepper motor problem

when i try to move my z axis manually in repetier-host or any other host software my z axis motors freeze up. The odd thing is when i have just one motor plugged(either the right or the left) into the board and i move it manually it seems to work fine. I tried re-downloading all my drivers. Im not sure what is left to do. My current EEPROM value for steps per mm on my z axis in the repetier-host is default at 4000, when i change the EEPROM z value for steps per mm to anything less than 2500 both motors work.

also they're standard nema 17 stepper motors, the maker is casun and the type is 42SHD0217-24B

I am using geeetech prusa i3 too. Maybe the power for z motors is not high enough. There is a screw on motor driver, rotate it to raise the voltage between that screw and GND. According to my experience, 1 V is enough.