There are many reasons this can happen. At times, it could be a construction defect, or installer error. Other times, the flue may have shifted in an earthquake, or a metal strap supporting a bend in the system could have failed. There are also instances where a chimney sweep unfamiliar with prefabricated metal fireplace systems may have disconnected the flue in the process of cleaning the flue, or removing the cap. In any of these cases, we are extremely knowledgeable and trained in repairs for getting your fireplace back to how it was intended.