When you make custom armour make sure your model was properly formatted. Eg the helmet cubes were in a group called head or helmet. Basically each part must be in its own group/folder. Then in mcreator when you select the custom model there will be a dropdown menu to select which group/folder will be rendered by that part of the armour. For the chestplate, the right and left plates are the shoulders.
When you make custom armour make sure your model was properly formatted. Eg the helmet cubes were in a group called head or helmet. Basically each part must be in its own group/folder. Then in mcreator when you select the custom model there will be a dropdown menu to select which group/folder will be rendered by that part of the armour. For the chestplate, the right and left plates are the shoulders.