It's a known occasional bug. I believe the only recourse is to reload an earlier save and try again. You might try leaving the cell, waiting three game days, and then retrying, but I have my doubts.
I will keep my eyes peeled for that one. Good points glargg, I know that leaving the cell, saving, loading from the main menu and returning to the cell, has helped with a couple of issues.
I could have mentioned that in my previous and otherwise pointless post, but as usual, old SR isn't playing with a full deck....anymore.