How are you testing your water and how often?

Are you using strips?
If so, you may have bleached them out!

Are you using DPD? If so, very high FC levels bleach out that test as well! Plus you should be testing your TC levels as well (Total Chlorine).

If you are using FAS-DPD to test your chlorine, you should be testing your CC levels as well (Combined Chloramines).

Since TC = FC + CC, in both cases you should see if you have a measurable CC level.

What is your pH level? That, too affects chlorine.