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...some sodium bromide may help, or better yet some bromine concentrate.
A few questions: Are your pH, Alkalinity, and hardness where they are supposed to be? What is in your shock and how much are you using? What is the setting on the feeder? Are the pucks eroding in the feeder? (They should over time.) If not, as Steve suggested, make sure the feeder isn't plugged. (Water needs to be able to wash through and pick up bromine.)The other possibility is an infection in the tub. The fact that your levels drop quickly makes me think that the bromine is getting used up trying to fight off an invader. Super-sanitizing (I prefer dichlor, but the the suggestions re: sodium bromide/bromine concentrate make sense, too -- see FLTUB's similar thread) until you are able to maintain a good residual is the next thing to do after checking the feeder.Good luck!
Even though my bromine ppm is seemingly always low... the weekly shock, ozone etc. keeps the water clear.