Anyone have experience with a leaking lannonstone chimney instead of brick

I have had some work done - new flashing, waterproofing )though I don't know what kind) The problem has gotten worse, especially with new flashings - replaced twice in 2 years. I have gone on the roof and used duct tape to tape up some of the flashing, which is the only thing that worked, but didn't hold. It is a 51 year old home, inherited from my father, and the chimney has always leaked, but just a little. There is a concrete crown, but it is flush to the chimney. How important is it to extend it beyond the chimney? The current crown is about 4 inches thick and is slanted, not flat, and has had a few caulking repair, but currently does not have any cracks, nor does the lannonstone on the chimney itself. Could this be the roof as it aligns with the chimney? Must I pull up the roofing to determine that and can the same roofing be reapplied? I know this is alot of info, but I am desperate.