Similar to the issue with Hod in the bugs described on this page, re Pete's remark here.
Sometimes she's just not at the mill during working hours. Used the mill, she didn't show up. Enter/exit her house she's not there.
Waited a bit still AWOL. Occasionally on a game reload she returns.
Worth a visit to Tracdown?
Edit: This might very well be a symptom of an in-game load & save.