I'm at 118.2, and I'm hunting Rotting Corpses (100 tactics). I figure I should be gaining. Why isn't it working?
My guess would be, that not all rotters spawn with 100 tactics, there is actually a range, so many would be under 100. In my experience you need to be within 25 points of the opponents tactics to gain. So, any rotters that spawned with less than 93.2, you would have no chance to gain from them. Leaving the ones that spawn with more than that to *potentially* gain from. Or, it could just be you need to turn something down.
Try something with 110 Tactics. I think you'll find it works better. If you happen to have one of the skill tutor statue crystal balls from the quest in Ter Mur, it will tell you if what you're fighting is too easy for you to get gains.
I don't know if it still works, it's a heck of a long time since I trained it, but I found that multiples worked better than singles. I gained by attacking 2 creatures at the same time.