Read the post here about "How To Get Started" as a mystery shopper under the Mystery Shopping Discussion section. There is a post with a link to a list (free) of some 250 mystery shopping companies to register with (also for free) and also a post with helpful advice for new shoppers about preparing to be a mystery shopper.
You will find that the "secret" to getting more shop invitations is to register with more companies and (if necessary) maybe "expand" the area that you will do shops in.
Try to find shops that are in the same general area and that can be done at the same time (check with your appropriate Scedular, when necessary, to see if a shop can be done on another day that better fits your schedule) to cut down on expenses.
By doing, at least, these things, you SHOULD see an increase in the number of shop offers that you receive.