Is your manga done digitally? (Not traditional, though can be printed) I think that might be the case, I suggest trying to replicate stuff as practice if it is. Another thing it might be, if you are contacting Japanese publishers, they usually don't like to publish American mangas because it might be seen as an "imposter" and won't be published or get them enough money. I think you should keep trying, you have good potential! I don't know any publishers though.
