Bromine could be being used up to fast. Did you add a bromine reserve at the beginning? If not (which I didn't do the first time), then your levels probably never got up to where they need to. When you first fill water, you need to bring bromine up to about 30ppm then shock it (to activate the bromine). Alternatively, to temporarily bring your levels up, assuming you have enough total bromine in the tub, use a shock. That will raise levels my burning off used up bromine and "freeing" it again, which will in turn raise your levels.
Maybe you don't have enough bromine in your tub, or maybe you don't shock it enough.
