Y-axis motors not running in sync

[regpye]

I am having a bit of a problem and can't work out how to fix it or what is causing it.
I have built a laser and have a strange problem that has developed.
The bed is wide 1500mm so i used two stepper motors, one at either side.
i have cloned the A on the CNC shield to be the second Y motor.
The movements are both in the same direction, but don't move in sync together, one is always a bit behind the other causing the gantry to twist.
i first thought that possibly it was cvaused through a loose timing wheel, but on checking, they are both tight. The belts are also tight.
Any ideas on what I might try to fix it?

[regpye]

Problem fixed. Found that the plug itto the motor driver had a bad connection. Luckily I didn't blow the driver. Refitted the connection and now it is working OK, the problem was that one motor wasn't working at all and the other motor was doing all the driving.

[Bertus Kruger]

Lucky you are.... These drivers tend to try an blow them self up to get the current up.