I've been importing raster files in ZMAP format using QGIS 2.4 and 2.6. Some of the files work fine but other files for some reason import a black image with nan as a default black and white value. Applying actual min and max values does nothing and shows the values in the raster file as zero.
The ZMAP files have all been exported equally from a gridding package, i.e. same XY min/max, header files look much the same, there are sensible values in all files, etc.
Any suggestions as to what might be causing this?