I shoot 2 fingers under and I always use one nock above and at the very least a rubber eliminator button below. Now I started using the rubber eliminator button right against the bottom of the arrow nock as a cushion and a brass string nock below the rubber button. Arrow nock slippage can be a real problem and really mess with ya. It will con you into thinking something else is wrong when it's just simply a nock slipping problem with a simple fix.
Besides, what's it gonna cost you to try one? Brass string nocks are cheap, try it and if it makes no difference just take it back off.
At least I can be sure my nock isn't slipping down so that eliminates one problem that I don't have to deal with, then look elsewhere for improper arrow flight issues like fletching contact, arrow spine, brace height, ect.