Schedulers' bonuses depend heavily on shop completion rates. Therefore, they may well assign first to shoppers that have proven track records. Second, some shops are only done quarterly. So, if they go on the Job board in Month 1, but are not actually due to the client until late in Month 3, the scheduler can wait to a "known" shopper to take it. Next, some clients may want, say 25 shops done this month, but will release 150 locations to the scheduler. As soon as the scheduler has 25 assigned, they may stop assigning others unless someone flakes.
So, many "rejections" are not so much rejections as issues of timing. If a scheduler can safely wait and be choosy about who gets the shop, a newbie may not be able to get it.
Advice... apply for lower pay shops with the same MSC; get a few; excel; move up to "preferred" shopper status to have better chances at the future "hard-to-land" assignments.