some agency owners i know (who shall remain nameless!) are racists and will try to screen out certain ethnic groups, or give them their B-list girls only.  unfortunately it's a fact of life and it's not always easy for a hobbyist to broach the subject

i've heard stories for example of a black guy being turned down by a girl who showed up on his door ("they didn't tell me you were black, i'm sorry, etc").  usually in this situation you're not obliged to pay her ANYTHING (not even cab fare)  so it's essentially just a waste of time.  i guess it doesn't hurt to tell the proprietor  of the agency that you are from a certain ethnic group and see what he says (it's just a shame that we live in a world where this sorta thing matters!)