How bad of a job do you have to do before getting demoted? I ask because my manager is constantly telling me how I'm not fast enough or work well enough in my department. How do you define "going fast" and "doing a good job" anyhow?
Go to this manager and politely ask for direction. Have them give you clear notes on exactly what they want from you, and if it seems unreasonable, tactfully point that out to them; provide suggestions on ways you might improve your performance.
I was told I was doing a great job. And I was still demoted. The DDP is supposed to guide you on what you are to do in that day. Ours does not count replenishment in though. I have found that no matter what you do, they expect you to do more on top of that. It's frustrating.
See if you can get your union steward to go up with you and talk to the manager about what exactly they want you to do.