The trunk lid hinge clips on 2007-2012 Nissan Sentras may be missing or the trunk is not flush with the fender. One or both of the stopper clips may be missing or out of position.
If you confirm that the trunk lid is not level with the fender, remove both trunk lid stopper clips (if missing, replace).
Apply Loctite Quick gel super glue between the stopper clips and hinges. Reattach the clips and hold in place until the adhesive sets.