The timing for it, you trying it online or in training?
Online I can NEVER do the 5D->DC->6A because of the lag.
In training and vs all I could say is as soon as the second hit connects dash cancel, wait a bit until you see ragna stops his animation (or see when specifically you can attack), then do the 6A. To know you are timing it correctly you can even do a carnage scissor special after the dash cancel.
It is really strange that after 5-6 hours of training you can't connect it =/