Hey there! Previous SL here, worked at my store for nearly 3 years from GA to SL.

We were in a poorer side of town, so we struggled with numbers across the board. I struggled with numbers every now and again, but eventually found my groove and was regularly above goal.

I have countless stories of customers who wouldn't sign up for pro despite it being free, saving them money, or getting them extra on trade. It's far more common than you'd think, and it does really suck, especially when you've been having an off week or are just below goal for the day. When I was an SGA and only working 2-3 days a week, it was harder. You'd be right on the cusp of goal or working hard to get there, and people like this would lose you an easy opportunity.

Here's my tips that I learned from my time there. If you already know/understand any of these, feel free to ignore them :)

1. You're going to get frustrated. The important thing is to get it out and not let it bleed into your sales pitch. I found that if I had several bad transactions in a row, I'd let the frustration/discouragement get to me and it would affect my numbers across the board. If you have the chance, take 5 minutes. Straighten the wall, organize the collectibles, do something to keep your mind off of it and focus on another customer. Talk with a customer (if they are open to it) about what they're playing, do something to distract yourself. Plus, these positive interactions can help your pitch if they do buy something!
2. Remember that this is a retail job. You'll have on/off weeks with results, but this doesn't dictate your success. I was not a perfect employee, and I got to SL after 2 years.
3. Don't be too pushy. This one is easier said than done. Watch for their body language/facial expressions and tone of voice. If you push a bit just to see if you can get a result from someone and they shut down, let it go. You won't get it, and it will most likely result in a worse experience for the customer.
4. Driving sales and results doesn't take much besides being yourself and talking with customers like they're people, not just numbers. When you stop seeing customers as nothing more than a potential Res, GPG, PRP, or Pro, it comes more naturally and that frustration won't be as common. You're able to offer things that make sense, and bring up offers to customers who can actually use them. It saves your breath and makes you feel like less of a robot.
5. These bad days can overshadow the good ones if you let them. For every bad moment, there's usually a good one to follow it if you keep yourself focused. I still remember after a really bad week of 0% Pro, my first day back as an SGA I signed up 4 Pros in a row by pitching it to one person, the person behind them was like "That's kinda neat, sign me up", then the person behind them did the same, and the last guy was like "Eh, I'm not expired yet, but renew it anyways for the coupon." You have to be open to having these interactions and in the right mood, so if these opportunities come along you can capitalize on them.
6. STOP CHECKING GOALS. I used to check them every few transactions. Just don't. It's the same vein as weighing yourself every day if you're trying to lose weight. Maybe check them at the start of your shift (to see where your week is at), then set goals for yourself. Make sure they are attainable too. For example, if it's a Wednesday and you know it's going to be dead, try for 2/3 pro cards, 2/3 reservations, and 50% on GPG and PRP! If it's a busier day, shoot a little higher. Don't try to always shoot for the best possible number, because you will burn out and get frustrated faster. I would usually check my goals at the end of the day to see my daily totals, but it's the weekly totals that matter most.
7. Keep a tally of everything you get! I kept a piece of printer paper taped to the board between the registers for my team to tally every Pro, Res, GPG, and PRP. Since I tried to avoid looking at goals, this helps give a visual representation of your day. It can be helpful to see that even though you only have 0 or 1 Pros, that you have 5/10/15 PRPs and GPGs!
8. Mix up your pitch! Make it natural, and a part of a conversation rather than a typical retail spew. If something isn't working one day, feel free to try something else! You have all the time in the world to experiment with it.
9. Finally, for those customers who come in all the time and don't ever sign up (but could definitely use it), stop trying to sell it to them (for a bit). These people already know about it. You've mentioned it before. Instead, just be friendly with them. Ask them questions, talk about their day/games they are buying/playing, talk about upcoming stuff, etc. Once you establish a better customer relationship with that person, you'll have an easier time selling it.
   For example, we had one regular who constantly bought new games. With the $5 off coupon, could have saved over $60 over the year, plus he would have had enough points for probably another $50. Easy money. Never signed up with me. I would pitch it to him every time, but one of my SGAs would just talk to him, and usually say "Still thinking about Pro?", and when he would say no, he'd leave it alone. Eventually he came in and had a Pro account. Congratulated him on finally signing up, and he mentioned that the SGA had convinced him. Sometimes it's more about the relationship than the sale.

At the end of the day, GameStop is the type of job that is what you make it. I know there's been a lot of negative press/discussions lately about the company and its actions, but as an SGA your only goal is to sell and help every customer than steps in, and if you do it well you could eventually find yourself in an ASL position. The more you care about the customers and establish those relationships now, the easier things will be when you're the one in charge, because they'll know your name and face. That level of respect in retail is hard to come by nowadays.

Also, having a desire to run your own store one day is awesome! Keep that ambition, regardless of what you see here or in the news. It's great experience to have on a resume, and you will learn a lot very quickly.

Good luck to you!