Can you explain that mechanic?
I did so in a subsequent post:
The mechanic is relatively simple - gaining souls puts that into your soul memory bank (regardless of whether or not you actually use them), and the game looks at that along with your actual soul level to gauge multiplayer ranges.
The problem arises when you play a decent amount of co-op. You attain a meaty amount of souls for helping someone beat a boss, especially in the mid to late game, and those souls go into your soul memory bank.
So you can see the problem...
Soul memory disappears in NG+ but in the normal game, where the player base is naturally larger, it can become a major problem. Eventually I ended up not being able to be summoned at all for anything, even very late-game bosses and encounters.