Y axis won't pull evenly

I just made a 4x3 3 axis CNC machine but I can’t get the y-axis to be pulled evenly. I tried putting the threaded rod in the middle but it still pulled one side more than the other. Any ideas?

Lots of details missing to really help, but it sounds like there more resistance on one side. When you give more details, we might be able to help more.

I have it rolling on bearings and the rod directly in the middle