Most likely is that you are not using enough chlorine to handle your bather load. I would shock to add more chlorine until you get to a situation where it holds. Then use more chlorine after your soak such that you get a measurement the next day.
The rough rule-of-thumb is that a hot (104ºF) tub needs around 5 fluid ounces of 6% bleach or 3-1/2 teaspoons of Dichlor or 7 teaspoons of non-chlorine shock (43% MPS) for every person-hour of soaking. This is just a starting point, however, since people sweat different amounts -- adjust your chlorine amount so that you measure 1-2 ppm FC before your next soak (assuming you are adding chlorine after your soak, which is what most people do). I also assume you have built up 30 ppm CYA by using Dichlor first.
It would also be useful to know how often you use the tub, if you have an ozonator, how you converted to BBB (i.e. did you drain/refill, did you do decontamination, use Spa System Flush, etc.?). Also, are you using a FAS-DPD chlorine test kit (i.e. TF-100 or K-2006) since a regular DPD chlorine test can bleach out at high chlorine levels?