Ethany
After you rule out organics with the Overnight Free chlorine loss test, then here are some other things to test.
Be sure the salt cell is clean. If you see calcium build up inside the cell, then clean per the manufacturers instructions.
If you are running the pump 24/7 with a 25K cell, then you may need to supplement the Zodiac with some liquid chlorine (regular 6% bleach).
1 gallon in a 20,000 gal pool should increase the FC from 2 to 5 and the Zodiac should be able to maintain that level unless you have a heavy bathing load.
PSG