OK...a little tough love here...this pool store is NOT helping you....so stop going there.

why aren't they testing your CYA? I assume you have none...and that is the first place to start....you need a decent amount of CYA in order to more effiently use your chlorine.

What are you using for shock? And what are you using for chlorine? If it were mem I would add 4 gallons of store nought 6% clorox and then take a reading yourself...then add 2 gallons everytime the FC level drops below 5ppm....just to see if you can sustain FC in the pool. You also should think about raising your PH....but right now you need to get and keep FC in the pool.