Soraka Ult Assists

I've noticed that about 15-20% of the time when I ult as Soraka, my ult doesn't give me an assist for kills that occur about 3-5 seconds after the ult. I understand the instant kill not really registering, but sometimes it's like I ult, a second passes, they die and I get no assist. I'm not sure why this is how it is, but it does effect my ability to get S ranks when I'm trying to help my team and never get credit for it. Lol.