Question
We purchased in August and notice a little bit of the flagstone rock from hottub that just falls off and did not worry much. Lately big chunks are coming off and falling into the pool, there has been freezing weather and some rain, andwe are notsure if this is causing the flagstone to come off in big chunks, the size of 1/2 piece of bread at most. Is this something to worry about?

Answer
I would worry.  Flagstone is a variable material, depending on its origin.  I spoke with a local quarry supplier, and he gets his from New York (we're in Nevada, so teh price is crazy expensive).  Its all in the source, and his is much more "dense" that other types.  

Most flagstone is a sedimentary rock, which means it was laid down as a lake or sea floor, over time.  As water works back into it and freezes, its like having little jack-hammers at work on the cracks inside the rock.  

It may also be that your mortar is failing...the cement/sand mixture thats holding the stone in place.

As to your problem, you cannot just "glue" these pieces back.  Structurally, the rock is probably still sound; it just looks ugly.

I'd get some bids from some pool contractors to replace this stone with new, more solid material, or another type of coping.  I just wnet to a pool show in Las Vegas, and there are literally hundreds to choose from.

Good luck ~M