The absolute best way to do that is to prove yourself by performance. As a new shopper with a company (and this happens with every company you sign up with, no matter how long you have shopped) you start off as a newbie. It may take a while to get your first job of any sort with them, but that is how you build your reputation. Chances are that the company already has a handful reliable shoppers in your area who will get awarded the work when they ask for it and are in rotation. So it may take a while to get the ones you want. If you are currently hitting a 50-50, you are doing better than most if you are relatively new to the company. And quite frankly, it may take a whole bunch of less than wonderful jobs done well to build up your reptuation to even be considered for the more desirable jobs.
Different companies handle this differently. Some companies allow you to self assign shops after you have completed a certain number of shops. Some don't even let you see jobs they do not think you have enough experience with them to perform. Some don't even take on new shoppers in areas where they currently are adequately staffed.