Template behaves strange and does not show up

I have an invoice which takes only one page A4. I have uploaded a template, but it behaves strangely.

When I set the template as a succeeding page it seems to work allright, i get two pages, the invoice and the succeeding page, and the latter shows the template.

When I set it as background template, I get one page with only the invoice, the template does not show up

When i set it as the leading page I get 3 pages, where the supposedly leading page is however second. On the first page is a part of the invoice (Which is an included view), the second page shows the template and on the last page, where the rest of the invoice is shown is also a copy of the included view.
But nowhere does the template function as background.

Any hints?

Many thanks