Z axis belt drive problem

Hello, everyone!

I'm building my first 3d printer and almost finished it (alu profile frame, 50x40x40cm, corexy -or H type- xy axis drive, 2 shaft z axis drive). And I have some problems with my Z-axis. I have one Nema17 and 2.5T belt to drive two 1204 shafts. Pic of its idea is attached.
Numbers are:
1 - motor
2 - two 1204 shafts, that hold bed
3 - group of idlers holding belt on corners
4 - tension idler, could be moved to regulate tension of belt
Problem is - each time I try to move Z axis, motor move few mm (or none at all) and that just make crunchy sound (like something inside is moving, but motor shaft is stand still).
i've tried to use only one shaft, changed other for idler, have the same effect. Changed both shafts for idlers (so it's no load, just belt with moving idlers) and still no success. Haven't tried another motor yet.
Btw, motors are these - [www.aliexpress.com] .
So is it just motor is to weak or is it faulty?
Had similar problem with xy axis (used two idler pulleys before motor for 90 degree turn), but solved it (just took that 90 degree turn away).

I realize this an old post but here are a few thoughts:

First, would be to make sure that the motor is not broken. Second, would be to make sure that the motion of all the pulleys isn't cause huge friction build up. Third, have you checked that everything is in proper alignment. Fourth, what are the specs of the belt drive; if you are bending it too tight then you can induce excess friction.

I would guess that the main problem is a faulty motor because there was no update, but if you are still having issues then try to reduce the number corners in the belt path. Personally, I would suggest just removing all the #3 idlers if this is just positioned under the bed.