Check the concentration of that Lowes Muriatic acid. I bought a gallon once, but it was much lower than the concentration of the pool store acid for nearly the same cost. I believe the concentration is given in deg Baume, which is actually the specific gravity, but it's not a one-to-one correlation to % HCL.

All I know is the pool store HCL fumes when opened (hold your breath!) and I need very little to make a difference. The lowes stuff didn't fume and it took a lot more to move my ph.