Perhaps there are some bricks missing or broken after a storm, or simply through normal wear and tear?
It is very important that you get the stack checked by a specialist such as Leonards & Airlievac. Never entrust chimney repairs to someone who isn't qualified and experienced. If the unthinkable were to happen and the chimney collapsed, you could be uninsured leading to a very expensive repair bill.
It is not only important that the stack is sound so there is no risk of a collapse, it is also vital that the flow is maintained for the efficiency of your fire or stove, and for safety.