If you haven't already sent it back, test the CYA levels first.

But in general:

=> If you have CYA < 20 ppm, you'll lose all your chlorine in a single day of sun -- regardless of your pool's chlorine level.
=> If you have CYA > 100 ppm, it will take several days for chlorine to drop noticeably -- unless you have algae or high swimmer load.