I have experienced something similar. I used to do a ton of the shops, now I do very few. I believe it has a lot to do with the stricter client requirements about spreading the shops out. Instead of assigning a ton of shops as soon as the new quarter begins, the schedulers have to ration them out on a weekly basis. I hardly ever plan more than a week in advance, so the quota is usually filled before I apply. It probably doesn't help the shops and reports are easier and pay more, making them more attractive to the competition.
My best advice would be to try to develop a relationship with the schedulers. Watch for when the shops come out, and apply for them right away, but also send an email directly to the schedulers. List the shops in which you are interested, and give a full range of dates on which you can commit to completing them, even if those dates fall outside the listed range. This will make it easier for the scheduler spread out the shops, and make you a more valuable shopper to them. Keep in mind different scenarios have different schedulers.
No guarantee this will work. In my area there was a huge area where the old MSC had trouble filling shops, now they fly off the board. No idea why.