I will add to that, that there are a couple of reasons they are not scheduled farther out, as a general rule.
We often have shoppers ask for long extensions, or for several small ones in a row, and then "flake" on us, leaving us with not enough time to get them completed for the client by another shopper. If it is someone that has done a large amount of shops for us and has always been very consistent with no "failed to complete" marks in their history, we are more likely to offer a more lengthy extension.
Also, some clients want shops spaced out between their locations a certain way. For example, maybe they want one per week per location. If an extension given to a shopper (even a great shopper) would cause us to not have a shop completed one week or to have more than one for a location one week, that could cause a problem with a client.
The most important thing to remember, by far, is to be communicative with your scheduler. If you can't do a shop, or if you need a little more time due to your circumstances, your scheduler should do all she/he can to work with you and help you be a successful shopper, with their power and authority (given by the client) to do so.
I hope that helps!
MaryACE
ACE Mystery Shopping