If you're stuck in A- and B+, you're probably good at certain aspects of the game but need to improve in other aspects. For example, you might have good awareness and aim but poor movement. To get to S rank, you need to identify any weaknesses you have and improve in those areas. Also, it helps to understand who your teammates are. Sometimes you will get stuck with a super defensive team who can't push the tower or secure the splat zones because they don't want to die, and sometimes you will get super offensive teammates who get a lot of kills and push the objective but fail to control the middle of the map or even your own base. If your teammates are mostly the former, then you might need to initiate pushing the objective and hope they follow. If your teammates are mostly the latter, then you might need to keep control over certain areas as well as ink over enemy ink so enemies can't swim to your base for free.
Also, don't use super jumps unless you need to be with your team immediately.
Depending on your weapon, you might want different gear.