Tribute not working on Spellthief's/Etc

I apologize if this is already a known issue.

When I buy Spellthief's Edge , sometimes (maybe 30% or more of the time), the Unique Passive - Tribute doesn't work. I main heimerdinger and I see my turrets hitting enemy champions all the time, and when I hit them with all my rockets, Tribute still doesn't "tick" and will still have 3 stacks sometimes. This occurs even when I'm attacking an enemy turret. This happens every single game when playing a support role, and anytime I but Spellthief's.

Is this a known bug?