Take a bucket of pool water and add chlorine to it. 1/4 teaspoon of 6% bleach in 2 gallons is 10 ppm FC. I'll bet you'll be able to have it register chlorine and hold it. If so, then you just haven't been adding enough to the pool and/or your chlorine generator may not be producing chlorine. You can take a water sample from the return when the SWCG is on to see if you register any chlorine.

It sounds like you've got nascent algae growth in the pool consuming the chlorine OR your SWCG isn't working.