Poor Instrumentation and/or Performances.

Once again "Instrumentation" is subjective, but Performances are not.

Are the instruments in tune within themselves AND with each of the other instruments beeing used.

Are the correct notes being played and are they in time with each other and also the other instruments as well. Freeform is Good, sloppy timing is not.

In this case Mastering your song will not help very much at all. It may even highlight the problems.