I just tried this myself and had the same issue. Test conditions:
70 Shaman with 3 points of Critical Affliction & 3 points of Improved Critical Affliction (20% chance).
Cast
Sicken.
I cast it 6 times, and out of the 8 time I cast it (15 ticks over time), not once did I get anything more than the standard 5 damage.
I'd dig deeper, but I guess Tallon/Vallon Zek is down or something, so there's several people on my server atm, so I can't insert some code & compile to debug it, lol.