Well, I had this exact same problem, & it turned out to be as simple as the A/C system needing to be recharged with freon. Don't know why low freon affects the front A/C but not the rear.
My front ac does not get cold. The rear unit works fine. I recall seeing a post with the same symptoms. I check the archives but could not find it. Can some one please point me in the right direction to correct this problem. Thank you.