The timing of the j.236D - j.214B(whiff) has to be perfect for 6C to land, the combo is also character specific cause against certain characters you have to delay the j.214B a bit to make it whiff, this in turn causes the 6C to be impossible to connect.
An easier variant is doing dash 2B - 2CC - 236C - 214B - 22B instead, though it does slightly less damage.