I've had this problem before. I was almost S rank (before I got it for the first time), and I was on a stream squadding with an S+ streamer. That day, I plummeted down from a high A+ to a high A-. Eventually I got it back, being able to outplay my rank, and eventually made it to S. However, my troubles aren't over. I am in the "get-and-lose" phase with my S rank. Now, don't think I suck, but many times, especially recently, my team just can't outplay the other team. There have been many games where I get a positive K-D ratio (e.g. 12 kills, 9 deaths), but my teammates are unable to hold off the opposing team when they're on a push when I'm down, offensive and defensive. There are other games where everything works out perfectly. What I'd suggest is having 2-3 main weapons that you like to use in a specific mode, and if you can double up, it saves you time for getting gear for it. What I've done is I've adopted the Tentatek and the Splat Rollers, and working on the Splooshes, Luna Blaster and Squiffers, then eventually the Splatterscopes. It all comes with practice, and setting small goals for yourself (to quote Rocket from his "10 ways to get better video" on YouTube). You reach each goal eventually, then continue to raise the bar a little bit each time. To take his example from the vid, he says, "Don't expect to be an S rank in a few days if what you're currently at is C+ 50." Now obviously we all know that this is a little farfetched, but it's still true. Continue to raise the bar higher and higher, little by little over time. Eventually you will be the player you want to be.
Now, I understand that you have certain weapons that you prefer, but part of being successful in ranked, especially, is to be able to use a variety of weapons. There are 2 ways to go about choosing a new weapon to learn:
1. See what a lot of people are using, and if you want to take advantage of it, try using that weapon on an alt account.
2. Look at the sub and special combos on each weapon. If you find a combo you want to use, experiment with that weapon a little bit.
I used method 1 with the Tentatek after 2.6.0 (correct me if I'm wrong, but that's the update that the 96 Deco's base damage got decreased from 62 to 52) because I thought it would be a really nice weapon to know how to use - good fire rate, and nice damage.
I used method 2 with my Luna, but I just didn't think the Luna Neo (where I got my inspiration from) suited me right. I like to be able to have a splash wall, or to be able to attack at short and long range.
Also, you might want to dedicate some weapons to Rainmaker mode ;)
Sorry for my long response, but hopefully this helped :)