"Track Device Pose" unstable in some device

My project has just one Vumark. I switch on "Track Device Pose", set "Tracking mode" as "Positional", and set "Fusion Mode" as "Optimize for Image Targets and VuMarks". I guess this is the way to enable the good old extending tracking. Now, if i test it in editor or in my iPad Air 2, the tracking is good. it keep tracking even the marker is covered or out of view. But if i tested it with my iPad Pro or iPhone 8, the position will quickly drift after the marker is covered or out of view. Is there any thing i can solve this problem? i am using unity 2018.2.1f1, the vuforia version is 7.5.20, Xcode 10.0