DiGenova and Toensing have always seemed upset about that and have mentioned several times that their guy wasn't interviewed. I did a quick search on Campbell to refresh my memory, and came across this Hill article from Dec 2017. Pic related was a nugget I'd forgotten, and might explain why he hasn't been interviewed. With all his documentation (reportedly 50K docs), they may not need a one-on-one interview anyway.
Here's the full article.
https://thehill.com/homenews/news/363019-doj-failed-to-interview-fbi-informant-before-it-filed-charges-in-russian