[GAMEPLAY] Not Getting Assists

So I've had two different games out of the last six or so, where I've hit the enemy champion, with either spells or AAs, or both, and an ally kills them, but the game doesn't give me an assist for some reason. First was with , I AA'd and Q'd and ally got them a couple seconds later, no assist. Second game, same thing: I was . I Q'd and hit them with passive, they died seconds later; nothing. Both times this happened in bot lane, too.