Hello, I often work with 4 viewports in the model space. A 3D view and 3 plan views.In one of the plan views Object Snap Tracking won't work. In the other 2 it works fine.The viewport where the problem occurs has TOP VIEW - its USC is the same as the WORLD USC.What can cause such a behaviour? Is there a setting or a system variable I should change?

Have you checked to see that you have OSnap ON in that viewport?  Or even different snap settings for that viewport?