Life leech not always working?

Looking at the setup there are definitely a few things to keep in mind.

You have almost no ailment protection, so getting chilled will slow your attack or frozen will prevent your attack and stop a leech

No boosts to accuracy are showing, so like they said above missing is definitely a potential problem, especially when fighting mobs that hit back enough for you to need the sustain from leeching.

Post Archnemesis league you have to worry about certain rare types. Not only do you have the ones you can't leech from, but the ones that can drain mana and will prevent you from using your skill button.