stepper motors on Z axis don't turn

Hello everybody,
My reprap is almost finished , my heatbed, my X, Y, and extruder motors work,.. The problem I have is the two stepper motors of the Z axis, they don't turn. Actually I first received a RAMPS 1.4 with only a four pin connection, to plug the two motors serial, but this RAMPS 1.4 didn't work, so I bought a new one that works well but the Z axis, on this new RAMPS, there is two 4 pins connections, to plug the Z motors in parallel ... When I try to activate the two motors I can hear little click once but they don't move, I tried several things like put a Z motor on X connection on Ramps, put the X motor on Z connection on Ramps,... When I did this I heard a bit of noise from the motors but still not moving. I read that I had to adjust the potentiometer on the stepstick, so the noise changes a bit but still not moving... and I tried to turn the potentiometer several turns in the both sides... according to what i read, this also could be something to adjust in the firmware..?? I use Sprinter...
If somebody knows the solution..?? it would be hugely appreciated, I can't wait to print )
Thanx
manu