How did you start your spa's chemical routine?
Typically when on bromine, you add a quantitiy of sodium bromide- which is different than the tablets that you might have. The sodium bromide I used was in granular form. A certain quantity based on your spa's water capacity, is added on startup or whenever you change the water. At the same time you add how ever many bromine tablets are called for to your floater or weir gate, or whatever dispenser you're using.
The sodium bromide gets into the water instantly. The tablets take a while to dissolve into the water.
That should have been discussed with you when they installed your spa. I'd ask your dealer for help- that's part of what you paid them for.
When I used bromine, I had a hard time regulating the readings. One day I'd get nothing, a couple days later the readings were off the charts.
It was very frustrating.