Have you tried additional shock? Whether it be a non-chlorine shock like MPS or a chlorine based one, I would try and shock it to see if maybe your water has some issues that are eating through your bromine too fast. I have a Marquis with the in line spa frog and went over a month on my first bromine cartridge with very little issues. Like you, I used the startup packet and had my cartridge set to 5. I had to turn it down to 3 to get the levels down. Since then, it has been fine.
Also, check your cartridge to make sure the little openings are actually open and dispensing bromine. Are you using a mineral cartridge as well?