Problem with Charactor Controller

I start to use PhysX one month ago and now I meet a problem with the behaviorCallback of charactor controller.

The PhysX version is 3.3.3

I create two CCTs, a Capsule and a box, with default desc. And then I reset some property like this:

capsuleCCT.stepOffset = 5;
capsuleCCT.height = 1
capsuleCCT.radius = 6

boxCCT.stepOffset = 5
boxCCT.halfHeight = 6
boxCCT.halfSide = 6
boxCCT.halfForward = 6

There is moving kinematic box actor in the scene just like SampleBrighe. Its height is 2.0f.

Two CCTs can do right thing when I return PxControllerBehaviorFlag::eCCT_CAN_RIDE_ON_OBJECT.

The capsule controller can do right thing ( I set its position on the box ) when I return PxControllerBehaviorFlag::eCCT_CAN_RIDE_ON_OBJECT | PxControllerBehaviorFlag::eCCT_SLIDE. But it can not climb on the box again after it falls from the box any more. It shakes at the box edge but never climb on it.

Do you know why the capsule controller do something wrong and how to fix it?

Thank you for your time!