Welcome to the forum and to mystery shopping, Liz. As walesmaven said, you seem to have a good understanding of the best way to find the jobs in your area. I would say keep doing what you are doing, keep signing up with companies, and keep checking the e-mail and Jobslinger.
If you've only been shopping two weeks, it sounds like you have made a really good start. Within your first 2 weeks, 25 shops AND signing up with companies, checking JobSlinger, you have found the forum, AND you have researched and planned and you GET IT about the job search and the competition. You are ahead of the game.
Jobs do come out at certain times of the month. Some companies posts periodically all month, but others throw all their jobs out at once. A lot of jobs with a lot of companies are usually posted within the last week of the month for action the following month. I expect many of my client companies will post August jobs between July 24 and July 30. Most will be grabbed immediately because of the competition. Be ready and be fast. Most companies do not have jobs sitting on the job board all month. There may be more companies that have jobs in your area - but you didn't see them because the jobs were already gone for the month by the time you registered and started checking.
Other companies post at different times, and, after working a couple of months, you will pick up on their timetables. For example, the MSC that posts my favorite lunch shop will throw those shops on the job board around 8 AM on approximately the 4th of August. They will send an e-mail about 11 AM. Those who are checking the job board rather than waiting for e-mail will have first choice. By 11 AM when the e-mail goes out, the best locations, if not ALL the locations, will either already be scheduled or will have several applications already sent to the scheduler.
Also - a new shopper doesn't necessarily see all the jobs. Build a good track record. Show that you are detailed and observant and you write a good report. Show that you are reliable. Many shops are e-mailed to me and never hit the job board. Not sure how helpful any of this is but just observations from an 8-year shopper who was "trained" by this forum.