Negative pressure will result from opening the door just once a day without utilizing an inline vent to force air out of the tent and prevent air from entering. Most growers generally won’t notice a problem until they begin sealing the tent’s seams and attempting to plug every tiny opening.
If the walls of your tent appear to be drawing inward, you are under negative pressure. The inline fan should be turned down and the zipper should adjust itself if you slightly open it.