So far, I have captured a soul from only two faces, and I need all ten. I was trying to soul capture another race, but the spell he gives to you didn't work. So I tried again on another race, and it still didn't work.
Why doesn't the spell work anymore?

http://www.uesp.net/wiki/Oblivion:Hermaeus_Mora
Possible workaround for this bug here, near bottom of page.