My DP04 has been having this issue ever since I bought it 70'000km ago. All tricks (bleeding clutch hydraulics, greasing axle of shifter, clutch lever fully extended) did not help. Only thing that helps is a quick increase in engine revs with clutch pulled and the gear shifts smoothly down. Upshifting is smooth always. This is IMHO related to the clutch drag. Nothing I could change ever, just got used to it.