There is a gap between my ADB and the floor or threshold. How do I adjust this?
To adjust the drop of the ADB, the unit must be properly installed on the door.
After installation, open the door, turn adjusting brass nut counter-clockwise a half turn.
Close the door and check the ADB for a proper seal.
Repeat these steps until a proper seal is achieved. 
Do not over-adjust.  In other words, back out the adjusting nut to the minimum needed to minimize force for the application. 

My floor is uneven and not allowing for a proper seal. What can I do to ensure a proper seal?
Zero ADBs can accommodate slight variations in the sealing surface. However, larger variations in the
floor surface can not be overcome by the ADB. A threshold may need to be properly installed to give a
flat surface to seal against.