Sellers using my brand name inspite of registered trademarks

I have a registered brand name with trademark and other required licenses for my brand. I have also registered the brand with amazon under the brand registry.

I still face the issue where other sellers are using my brand name and tagging my products. They are selling a poor quality product at a lower price which is affecting my sales and brand reputation.

I have mailed to amazon seller about this issue but they didn’t seem to take any action on this.

Any help regarding this matter from you will be quite helpful.

Thanks.