Often times bad breath can signal other problems in the body. Sometimes food can be a contributing factor (have you changed diet recently). Dental disease is a common occurrence in some breeds and is a life-long ongoing problem.
Pretty wishy washy answer...
Our Kaylee has chronic dental disease. Her last dental - about a year ago had results (clean teeth and "sweet" breath) for about 3 months. She's back to her bad breath.
I've had cases where the chronic infection was successfully treated with pulse antibiotic therapy, oral rinses. There are new dental treatments where application to the teeth is supposed to reduce plaque and gingivitis.
Bottom line, probably best to check with your vet. They are best at evaluating and isolating then treating problems. There are situations where managing the situation is about the best we can do, no cure.
Doug & Sandy
Kaylee
Winnie 6 1/2 year old golden
2008 Southwind 2009 Honda CRV