Their particular client must approve every shopper who does their shops. There has been some discussion about how and why some shoppers are approved and others are not, but I don't think anyone has a definitive answer. Sometimes it is just a matter of time before a shopper is approved for this client. Don't let it get to you. You didn't do anything wrong and as far as I can tell there is nothing you can do to get yourself approved. Just be patient and look for other opportunities in the meantime.