Blades don't turn if the glass jar is on

I have an oster blender, when I turn it on, it works. If I put the base with the blade assembly without the glass jar and turn on , it works and the blades rotates. Once I place the jar and I make sure it's tightened to the base and the blades are in place and then turn the blender on, I just hear a humming sound but the blades are not rotating.