I think I used to know how to fix this but my technique isn't working.  Seems to me I could just go into the material room and click the Image_Source and then the hair would reappear.  But it's not working with Sensual hair for V4.  I just hacked the CR2 to change all the displays to TEXTURESHADED so that's not it.  It shows up fine, then I apply a MAT pose and it disappears.  It's still there, just invisible.  Arg.

What I do when that happens is to go to the material room and turn the transparency dial. Before leaving the material room I put the transparency back the way it was.

Sometimes even just going into the Material Room and picking different material groups will set the display right again. I keep meaning to have a look into one of the hair models that has this glitch, and compare it with a normal one and see if there's anything in the HR2 that might be behind the error (rather than it being something weird about the texture file).

I had the same problem with Sensual Hair.  I believe I went into the Materials Room and clicked on Remove Detached Nodes.  That seemed to do the trick.

Hope this helps. ^_^

Wow! Talk about a complicated way to do something easy!!

Make sure your hair is selected!

Top Menu Bar: **Display/Element Style/Wireframe **          Job done!

The rest of your scene dispalys as normal, the hair is displayed in wireframe, it's actually easier to see where it falls and easier than going into the material room!!

I use the flat shading, and if that doesn't work, while posiing I set the (material) transparency to 0 while I pose :)

It was the Blinn shader.  When I disconnected that it showed up fine (none of the suggestions were working).  Thanks all.

Whenever the hair disappears after applying a pose, I go into the Material Room and check "Normals Forward" for all the hair parts. It then reappears in the preview window.