I have been mystery shopping for 9 months and am not certified, and do between 40 and 50 shops a month. I don't think that being certified helps you get shops, but when you are assigned for a shop and follow through is what counts. In the 9 months shopping I have only missed 3 shops, because of a work related issue and the schedulers were more than happy to re-schedule the shop for me.
The most impoirtant thing in mystery shopping is if assigned a shop complete it, but if you find that you can't let the schedulers know.
Mike