LuLu not able to cast ultimate while rooted

Hey, i'm a support main and i've been playing lulu for a good 3 months now. I love playing lulu so much but about a month ago, i realized that when i got rooted, i couldn't cast my ultimate to save me or anyone else. I took it into custom games to see if it probally was just my eyes and i'm not quick enough but even with all my friends playing lulu and me rooting them, they couldn't use the ultimate either. Although people might think that this is something thats useless, it's actually very important because lulu's ultimate is used for extra health and a knockup and if you get rooted it's supposed to save either you or your teamate. expecially if it's your adc. Anways, if you have any feedback post below.