A bit more info would be usefull, like some details of the construction etc.
But by that work of art xD i would suggest some possible reasons
One would be one or both motors skips steps, or they are not "in tune" with eachother, or, one does not meet the torque requirements or the gear ratios / steps / unit are not well defined etc.
There could also be mechanical problems.
A quick snap of your configuration would not hurt, to see your current parameters and such