That's because for some reason the EXIF data for all shots appears to be the same (ISO 100, 1/320, f/7.1). Photoshop sees this and doesn't know what to do about it. This typically occurs if you try and HDR different exposures of the same photograph obtained from a RAW processor or similar - images that are differently exposed in the camera should have different EXIF values and work fine, so I don't know what's going on here.
Anyway, you can use
Exifer (free) to strip the EXIF data out of the photos, at which point Photoshop should revert to its (less accurate, but still good enough) 'look at the image and figure out the exposure' routine, which will work fine.