Xerath mana auto attack passive is not working

I just played a match where I was Xerath and only 1 out of 3 maybe 1 out of 4 of his auto attacks would trigger his mana return on auto attack hit.

Mid to late game it didn't trigger at all, which meant I had to buy extra dorans rings in the beginning just to keep mana.

If any one else has noticed this problem let your concern be heard.