Sometimes the drop down is drawn for just a part of the second and disappears immediately. After that the drop down opens as expected. The attached video demonstrates the behavior.

this.radDropDownList1.DropDownAnimationEnabled = true;
this.radDropDownList1.DropDownAnimationFrames = 35;
this.radDropDownList1.DropDownAnimationEasing = RadEasingType.InOutQuad;

Workaround: you can either disable the animation or decrease the DropDownAnimationFrames.