Am I using Velocity incorrectly?

It's so weird, I'm trying to make the player's character "jump" high when you click a button by changing the velocity in it's HumanoidRootPart, but, it only works sometimes? Am I using veloctiy wrong?

Only put the part of the script in question

            player.Character.HumanoidRootPart.Velocity=Vector3.new(0,100,0)