Background image doesn´t appear when I download .pdf

[Lirorix]

I placed an image at the back, and layered grids over it with writing on top of these white grids.

This has worked fine before, but now with a new page I created, the background image does not appear when I download the .pdf document.

It DOES appear if I download the page as a .png file.

I have tried setting the image as 'background image', as well as simply placing it as the lowest element. Nothing seems to be working.

[Lirorix]

(the blue line around the document is the image behind text and white grids that I want to appear in the .pdf)