Nunu's snowballs have a travel time and a cast time, and you can still cast it an instant before willump's turn is.
This means that it's probably that willump's turn will come in and nunu will still be launching snowballs, and it's probably that those snowballs will not get their snowbound into effect because Willump already used his turn.
Increase Willump's turn time from 3 seconds to 3.15 - 3.5 seconds (the minimum / maximum amount of time for Nunu to hit an enemy with his snowballs at maximum distance) **only** if he sees Nunu is still launching snowballs.
Decrease Nunu's time interval to launch his snowballs from 3 seconds to 2.85 - 2.5 seconds.
I don't like the last one since it's a nerf but it would still fix this issue.
It's not nice **not** being able to root enemies after hitting them with snowballs with the same cast of the ability (see video ↑).
I'm sure Nunu would be angry if he saw Willump going in during Nunu's turn. You **can't** go in someone else's turn!!