Hi Zhu,
So you cannot scroll up or down when your screen displays some horizontal slider. I personally cannot reproduce it on my mobile devices. But I experienced this "issue" several times on other sites with horizontal sliders, but I was able to scroll up/down after several touch attempts. I don't think that this "issue" is somehow possible to fix. The only solution in your case is to disable horizontal slider and use static images. Of course, you can contact bxslider support, but I don't think that they also can do anything here.