I've been mystery shopping for several years with a couple of different companies. I've found that iSecretShop has been the most reliable and consistent with interesting and fun shops. As for being paid, I honestly don't do it for the money, it's just fun to do and I get to meet a lot of very interesting people in numerous varied industries. As for being offered upscale shopping opportunities, I've found that those were usually offered by another company but iSecretShop is still my "go to" source when I'm feeling I need to "explore" the community.
It's understandable to barred if any shops weren't completed per instructions. Over the years I've found that if you have a valid reason you cannot fulfill or complete an obligation, many times providing the contracting source this information will allow you to make it up or take a leave of absence.