guardian is still unreliable as ever

I took a little break from lol. I came back after hearing about aatrox's rework, and the new fad of duskstorm combo. I started doing missions, and I felt like going old school with one of my favorite champs (taric). choose guardian it says it can be set up after healing, or shielding an ally champ for 2.5 seconds. I use bastion to shield an ally from a tower shot it didn't kill them, but i noticed my guardian didn't go off wtf is going on.