Kalista W Sentinel Confusion

Hey. i love playing kalista she's actually really fun. But i am getting very confused on kalista's W active, her sentinels. When i am in laning phase with her, i usually throw a sentinel into the river just incase of ganks, or into the tri bush to supply additional vision. but for some dumb reason, and i dont know if im doing anything wrong. whenever i throw a sentinel anywhere, it just travels a short distance and turns back in a straight line and wanders around back and forth, not even roaming like it should be (like if i threw it in river, it'd go from the entrance to near the entrance where the exploding plant and vision plant is, then turn back, regardless of what rank it is, and never actually roam like it should.) and i'm watching these pro players on youtube who are playing kalista and theirs freely roam, even at rank 1. what's going on here and how do i make it roam. it's confusing the living hell out of me.