Were you playing in a "core" lobby by any chance? A lot of people who had this problem said it never worked for them.
When I completed the challenge it was right after selecting it in the ToW menu and loading into Fjell - I wasn't squad leader right away but took it off our leader because he wasn't marking any objectives and managed 3 kills.
This issues seems to happen on Grand Operations where it teverts to buddy heals and wont track any Tides of war achievements as well as the Launch Operation assignment.
My squad and I had this problem, in conquest on fjell. Our Squad leader got it but no one else.
