I think that John is correct. "One" other possible problem is moisture.......public enemy number one.............and it can do it to your brick real fast. If you "have" soft brick, which should have never been installed to begin with, the face will blow right off leaving the soft interior which will go right back to dust.
Regardless of what the problem is, John's advice is good, replace it. If there is other issues further down the flue, the liner "combined" with the new stack could be necessary.
Like I always say, get info from contractors and repost. The more information people have , the more they can help.