Why meshparts in the player character get their positions and rotations messed up?

I have a model (called "StarterCharacter") which I have put in the StarterPlayer folder to replace the original human model. In this model I have different MeshParts (items such as hats...) that I got from the Marketplace and I have positioned and rotated them so that they "fit" the model. When I run the game their position and rotation get messed up... Why and how can I fix this?