Andrea Allred goes searching for the truth:
I truly don’t know when it happened, but over the last while I have noticed that the percentage complete on indexes has disappeared when I run sp_whoisactive. It makes me so sad! I used that functionality often to track how things were progressing in my databases. At first I thought it was a version thing and it would come back, then I wondered if it was only when I use “ONLINE = ON”, but I am seeing it blank more and more. It has left me feeling like I am missing something and today, I finally did the digging to learn how to get that visibility back.
Click through to see how.