Lulu's polymorph does not work 100% of the time.

I was in a rank game using Lulu, when i try to polymorph the enemy ahri when she is using her ult the polymorph didn't work on her even when she was engaging with her ult. I dont know if its in mid dash but ahris dash shouldnt cancel polymorph and it hasnt in the past. I also tried to polymorph the enemy pantheon and he did not polymorph, he was able to keep attacking the turret, it sometimes appears that he can block it with his passive. I had pantheon jump on me but miss me, when i polymorph him he kept the same movement speed and his animation looked like he was polymorphed but he didn't seem like it worked. It might be because of the jump at the same time i polymorphed him that made it cancel the polymorphs debuffs but not the animation on him. It might be the fact of polymorphing certain champions when they are in mid actions seem to either cancel polymorph or cancel its debuffs (ie keep their movement speed).