There is a technical limitation with the contact widget where two maps cannot be displayed on the same page. Since you already have a contact widget in your header (but hidden via CSS) that’s what’s causing the issue.
A workaround would be to keep the header with the contact widget displayed on the contact page.
Another option would be to use a text widget instead of the contact widget in your sidebar on the contact page. Within the text widget, you could place your address, and provide a link to the Google map, instead of embedding the map right on the page. Google maps provides a direct link to any map via their site.