Photoshop: Magic wand broken? (eyedropper sampling setting)

(Photoshop CS4)

Magic wand selection is failing to distinguish between white and black (I am a veteran user).

I am trying to select a small section of a greyscale drawing (architectural drawing, 2D CAD export RGB tiff, no compression, no anti alias, white fills, black lines) to make masks for my 2D material (photo) fills.

No matter how often I reset the magic wand tool to 'factory settings', on my PC it just won't work. On a different PC, exact same configuration, exact same software, exact same file, it works like a charm.

Extremely frustrating. Like trying to hit a moving nail with a hammer and only hitting wood.

As long as the area I try to select is big enough (e.g. more than 15 pixels) it will work. Selecting an area of less than 10 pixels will fail.