It could be some things. First, remove the door closer and see if the door shuts with little effort or needs some force to close fully. If the door does not swing well, or a bent hinge or sprung frame causes resistance, these factors can significantly inhibit the closer's ability to close and latch the door. Sometimes the solution is to fix or replace the door and/or door frame.
Air pressure can also be a factor, as I mentioned in the article. Significant air pressure that opposes the motion of the closer can be a severe problem to solve.
If the door swings well and there is no air pressure problem, it sounds to me like the arm may not be installed correctly, or perhaps the closer is not installed correctly. If you can download the manufacturer's install instructions, you can check the mounting hole locations and see if they are right. Also if the arm is installed at the wrong angle on the spindle, the closer may not work correctly.