Pilot hole the marks.
Gap under new front door.
Make marks through the pre drilled attachment holes onto the door proper.
If you have a large gap at the bottom of your door that may be helping the door to stay plumb.
A drafty door with chilly drafts air blowing through your entryway.
Open the door and place a wooden block or hardback book under the outer corner of the door to hold it in place while you work.
Water leaking around the door or water leaking under the door or door bottom.
A door gap resulting in light under the door or peeking around the edges of the door.
As long as the gap is even on an interior door it shouldn t be why a door isn t closing properly.
Be careful not to overdo it and seal the door so that it can t settle with the rest of the house.
Place the sweep at the bottom of the door and align the sweep edge so that it blocks the door gap.