Any electric brake experts here?

Hey guys. I had a Dexter "never adjust" brake come apart last December. A trailer repair shop told me that was a common problem and I should replace all my brakes with manually adjustable brakes. He sold me brakes rated for 5.2 to 6K and said they were the same brakes as the ones for 7K. They are 12" X 2" like I had, but they don't have much stopping power. Could it be that the magnet does not have as much magnetism as the 7K magnets? I wish I saved the old ones, but I did not. I called the distributer where I got the brakes and they tried to tell me the drums were worn and that magnets are not the problem. I have less than 5,000 miles on the drums and no noticeable wear. Granted, I didn't measure them with a mic, but I've seen enough brakes in my day to know they aren't worn beyond spec. Also, I always have my exhaust brake on when driving and try not to overwork the brakes.
Before changing the brakes, I ran my controller at about 4. Now, I have it on 10 and cannot lock the brakes even when manually applying them. They are all properly adjusted for a light "scrubbing" and all magnets work. While jacked up, I spun each wheel and had my wife apply the brakes and each locked up.

What are y'alls thoughts on this?