The first ten shops shoppers do for Maritz must be scheduled by the schedulers. You must request and they will call you and schedule you. Once you complete ten shops, you will be able to self-schedule.
You are not doing anything wrong. The problem is that anybody who has successfully completed 10 shops or more can self-schedule. You must request the shop and wait to be called. Maritz typically waits a couple of days to give their currently-successful shoppers first chance at the shops. Any shops that are really good will probably be snapped up by shoppers with self-schedule before the Maritz staff gets back to you. My advice would be to request a couple of the less desirable shops. When the scheduler calls to schedule you, agree to take as many undesirable shops as you can in order to complete the required 10 to get self schedule. Then you can go in and take the shops you want when you want them.