This problem may be caused by not setting the position of the monitor correctly in the system.
Here are the steps on how to correctly set the monitor position:
1. Please make sure that you have connected our Docking Station (DS808/DS806) according to our instructions, and please make sure that you have successfully installed the corresponding driver (DS808: Displaylink Manager) (DS806: Instantview)
2. Please find and click on the Apple icon in the upper left corner of your Mac, and click System Settings.
3. Find and click on the Displays option.
4. Click the Arrange option, then you can adjust the screen position in this option.
5. Image before drag and adjust.
6. Just use the mouse to click and hold the screen you want to reposition and drag. Align the bottom of all screens on the same horizontal line. And click "Done" (image after adjust)
After completing this setup, you should be able to smoothly move the mouse or drag and drop files between multiple screens.