add_meta_box does not go ‘side’

I’m trying to add some metabox to my custom post type. So I add this:

add_meta_box(
    'my_custom_meta_box_id', 
    'My-meta-box-title',
    'my_custom_meta_box_callback', 
    'my-custom-post-type', 
    'side', 
    'default'
);

So the meta-box keep showing at bottom left. What could cause that problem?

This might be caused because you have tried to move your custom meta box to some place and WordPress respects the user’s ordering/positioning of these boxes. You can do as gmazzap has told because this will create a new user with blank preferences.

Or you can login to your database, find DBPREFIX_usermeta (DBPREFIX is your database prefix) and find related database entries. In my case:

You can see meta-box-order_PLUGINNAME (where it is your plugin’s name) and screen_layout_PLUGINNAME. When I removed these entries, the problem was gone.