cs 5.5 illustrator "recolor artwork" tool stopped working

For the past several days, I've been repeating the following steps with no problem:

1. open PDF in Illustrator 5.5
2. change document color mode to CMYK color
3. select all
4. Choose "Recolor Artwork"
5. adjust sliders to c:0% m:0% y:0% k:100%
6. Save PDF with new color

Today, when I try to do the exact same thing, everything goes as planned EXCEPT, the 'recolor artwork' command doesn't 'take'. 

I change the values, but upon completion, the artwork has not changed colors. Please advise.  What setting have I jacked up?

Thanks!

Jeff