Cases like these are always security related. Double check your security settings (what is your user role, and is that role allowed to change the entity. Is there a generalization with different security settings messing things up?).
If Ronald's suggestion doesn't correct it, make sure the dataview has an object to display. For instance, if the dataview is on an edit page opened by a New button on a datagrid, Mendix will create the object for you. However, if the dataview is on a page that doesn't get opened from a datagrid or other Mendix control like a TemplateGrid, I have not created the object that the dataview should display, so it has nowhere for me to enter data.