Hi,
I have a Riolu that I have trained at least 20 levels with a soothe bell (and I'm so use to training without looking at this point) that I realized only around 95 my happiness was going up by +2 instead of +10. I haven't traded it, only fainted once early on so those shouldn't be factors. EV reducing berries won't increase happiness and i don't train with it in the first slot since I switch train it.