A few times on the 5v5 map I went to fling a Sivir and she just wouldn't get picked up. It did the damage but she twitched for a second and was not moved. (NO spell shield was not on.)

I've played Sivir quite a bit and never had this issue and was always flung with Fling. That was kinda fun to say... Anywho... Are you sure you weren't lagging...? Or were you close to the edge of a map or something? Maybe she ran into the boundary of the map and rubber-banded back to the original spot...?

Bump for same issue, I'd be trying to fling sivir and she wouldnt move and the spell was used/put on cooldown.