Make sure all your pigtails inside the gang box are connected securely. I had this issue once and it ended up being a few loose neutral wires in the pigtail.
As some have stated, I'd start with a mokapot and a better grinder. If you don't want to spend hundreds on a good grinder, then kingrinders k6 manual grinder is amazing for about 120. Basically a over engineerinered peppermill.
Make sure all your pigtails inside the gang box are connected securely. I had this issue once and it ended up being a few loose neutral wires in the pigtail.