QGIS print composer export excludes one layer

I am trying to export a map from QGIS print composer but the resulting files (I've tried all image, .svg, and .pdf formats) do not appear as the map did in the composer.

It is as if one of the raster layers (an elevation model) has just not been included although the other raster layer (a hillshade layer) has worked. Every other layer, all vectors, appear fine.

Below is a screenshot of how it looks in the print composer. The topography colouring is from the elevation model raster layer, the hillshade is a semi-transparent layer above it.

Here is what the resulting export looks like. This is also what it would look like if the elevation raster wasn't present at all.

Has anyone come across this problem before? I'm using QGIS 2.6.1 but have also tried the recent 2.7 dev version.