So I just discover a problem in the bending modifier. When I select some lined vertex and add the bending modifier, the vertex can bend perfectly, but when I select some other lined vertex (same model) and try to bend it, the vertex can't bend properly, it only bend a little even that I have put the angle higher than the first one. Plus, the gizmo only showed as a thin single line, not a block-like gizmo in normal bending modifier.
I've attached a pic for my problem. As you can see, the first two pic is how bending works normally and the last two is the problem. You see that the vertex won't bend even that the angle is higher than the normal one. The gizmo in the last two even only shown as a thin single line, and rotating and resizing the gizmo don't help much.
Your selection must be greater than 0 in direction you want to bend. In your case it would be x. Just move one of your selected vertex slightly in x direction and then it should work.