I used to fly with China Airlines quite often, and a common trend on virtually all of their flights was significant and sustained turbulence. Perhaps their pilots aren't trained as adequately or in the same method as pilots for other airlines?
I've never flown CA so I don't know how they operate but I know out of Japan to North America you can get stuck at a unfavourable altitude and bump all the way across the Pacific. If there is an aircraft above you and one below you on your track, you getting bumped around and there is not much you can do about it, if for example your trying to get above or below the trop or out of the jetstream.
Sometimes you can negotiate an altitude change by talking to aircraft around you and coordinating an altitude change and sometimes not.
I do find that some crews are less likely to 'negotiate' on 123.45