Bear in mind as well that possibly they are in the same position as the factory owner in that he may have promised the bonus to the agency and renaged on that
Regardless I doubt very much if you have a case against the factory owner as you were never employed by them.
As for the agency, again, you have nothing in writing and could end up in a "he said, she said" type arguement. Did you sign an agreement with the agency and did it mention this in any way
I doubt very much that the agency is working on a 10% margin.
Might be worth asking the factory has the agency charged them for the bonus, if they have, then basically they are increasing their profit.