Z Axis Stepper Motors won't move

Hello RepRappers,
I have built my own 3d printer and last week I began testing it out. I haven't had any problems with the X and Y axes but the Z stepper motors won't working correctly.
After a bit of trial and error, I managed to make them work, individually. When one or the other is connected on its own to the Z-axis stepper motor, it works correctly, with no problems.
Though when I try and plug in both the motors they make a buzzing noise and vibrate as if they were trying to move but they stay still. I remove the X-axis from the motors and tested them with no extra bits, only the steppers. This way they move together but when I add the couplers - which then connect to the lead screw - they stop moving.
I don't think it is a structural problem, sounds like it has something to do with the current - do I have to tweak the pot on the driver? - or with the firmware.
I'd appreciate any kind of help. Don't hesitate to ask for more info.
Thank you in advance!

If you have connected the two motors in parallel (which is not usually the best way, but is how RAMPS and some other budget electronics connect two Z motor connectors) then they are probably not getting enough current. Try turning up the motor current, in firmware if your electronics supports that, or the pot on the driver module if it doesn't. If you can't get enough current without the driver overheating, then connect the two motors in series instead.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Thank you for your reply,
I will try as soon as possible, is there any way to change the motor current using Marlin?
If not, I'll try using the pot on the driver.

If the driver has a pot then you won't be able to set the current in firmware.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

...what dc42 said, but also check firmware for a decent max. Z_feedrate. I had to reduce mine from 5 mm/s to 2mm/s, that's related to the fine pitch of M5 leadscrews.

I have finally solved the problem,
I changed the current delivered to the motor by moving the pot on the driver, I set the max feed rate to 2 mm/s and I tweaked the structure to make the motors more comfortable.
Thanks to everyone that helped!

All this explained in the "manual" Why people don't bother to read, reread eventually, the available doc ?

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"A comical prototype doesn't mean a dumb idea is possible" (Thunderf00t)