I've got the same scanner and had a problem with 
pinks shades on one cel.. they appeared white 
also & no amount of tweaking made a difference.

It was the only time pink on a cel gave me 
trouble.

After tweaking & no results i simply gave up!  
Maybe you could contact Canon and ask them.  I 
wasn't too bothered as the cel was not really 
important to me.  

Normally i'd advise whacking it.. but this thing 
is too delicate!!