All assignments are done through email, so you have to be one of the first to respond, have a good work history, and know the ropes in order to get an assignment. The email doesn't have all the restrictions, such as no more than 1 shop per day, so those who have done the shop before have an advantage.
The schedulers are very responsive. I'm guessing there are a lot of shoppers in your area, and by the time you submitted your request and were notified that all 4 couldn't be done the same day, that all the shops were already assigned. That happened to me, too. Someone (or several someones) who were familiar with the shops knew the shops couldn't be done on the same day, and they requested each shop on a different day, so they got the assignments. You'll know for next time.
FWIW, this company's shops and reports are easy and straight-forward. The schedulers are nice. It's worth hanging in there.