Possible problems that occur with the door closers are as follows:
Door will close too fast and bang off frame
Door closer will have oil leaking form it
After adjusting door closer it will only work correctly for a short time
Door swings past its central point and does not meet in the middle as it did before
Above listed problems would indicate that the door closer would need replacing.
Other possible problems are
The arm could be worn or needs replacing which keeps the door in line on the top
The shoe is on the bottom of the door this keeps the door in line on the bottom
Window repair.ie supply, fit and repair door closers to all types of doors.