This happened to me once, and I finally got an answer from them:
You have a limit on the number of shops that you can have assigned in any given month. This limit only applies to self assigned shops. The available shops will disappear when you are at your limit. Once you have completed a couple of shops, then the available shops will re-appear.
If you have a route that they assign to you, it will not affect your ability to self assign to the same limit as when there is no route. (So you can do lots more shops if they assign them than when you self assign them????)
I tested out what they told me, and the shops showed back up as soon as I completed a couple that I had scheduled.
They also told me that the number of shops that you can have assigned in a month will differ based on your history with them.