Practice makes perfect?

When we begin anything, running, swimming, dancing, playing an instrument; we are told to practice every day in order to improve.  However, what if practicing certain  things every day doesn't seem to help one improve?  What if what someone needs is a 'break' from whatever it is that is not improving, and come back to it in time?  Is that a bad thing, or is there really something to be said for those who push through times of difficulty that seem insurmountable? What makes these people so much more admired or liked because they are able to not get frustrated with themselves or their abilities? 

Case in point:  I am practicing orchestral excerpts for an upcoming audition.  However, no matter what i do, i seem to have a mental block against one of the said excerpts.  How do i go about overcoming this, when just playing it every day seems to do no good?  'Practice makes perfect' keeps ringing in my mind, while wrong notes are ringing in my ears.  

Any suggestions?  Maybe just a day away from it will help to make the piece seem more playable.  Yet in these situations, with an audition quickly approaching, how can one take a day off?  Is that really possible... no.  So yet again I am back to wondering how to just "make it work," as Tim Gunn would say.

For all of you musicians out there, whether student, armature, or professional... what do you do?  How do you 'clear the mechanism'?