After adjusting the brakes it may take several hundred miles before the brake shoes wear so that they make full contact with the drums.
Then another brake adjustment may be needed.
Also you may have the same problem I had with a new trailer. When the axles were assembled at the factory (which in my case was in China) they damaged the grease seals and I found three of my brakes covered with grease.
After I cleaned up the mess and replaced all four brake assemblies I had good brakes. I did all four to be sure I would have balanced braking.