You need to convert background to a layer first. Right click on background for options.
Sorry, I think I didn't explain that well... I am not using the "backgroud layer". Just 2 images in 2 different layers. I wanted one of them to be first plan, and the other second plan. But when I move the layers of the one I wanted as second plan down the image desappears instead of appearing behind the other one...
A top layer needs to contain transparent or semi-transparent pixels, or have less than 100% opacity, or be partially masked, or have a special blending mode for a lower layer(s) to be visible. Does your top layer have any of these attributes?
You can increase the canvas size double it height or width and move images layers so they show side by side or one over the other.
You can add a layer mask to the top image layer and paint with black on the layer mask where you want the lower image to show through to make a composite.
You can lower the opacity on the top layer to make it look like a film double exposure.
What you are seeing is a 100% opaque image covering the other what you see is the image higher in the layer stack