Just out of curiosity, how are you trying to do it? THere are two different ways to do it, and on SFIV, it’s actually even easier now because of the input shortcuts.
The two most common ways is doing the C.HP in the middle of the DP code. However, this can cause people some problem. I prefer to do the DP code like this when using this method:
By adding the one down/back part of the DP code, it gives you two joystick positions to hit Fierce. Just try to hit Fierce at :db: and then quickly go to :df: + :k:.
The other way to do it is to hold :df: and hit Fierce, and then immediately do a very small counter-clockwise circle back to :df: and hit :k:. The circle you do on the stick should not actually go around the whole stick. The top-left part of the circle should barely go outside of the neutral position. Small circle.
If you are coming out with a Super instead, it’s because you are trying the second method, but starting the stick at :d: instead of :df:. Also, another thing that might help is to make sure you end at :df:. Whenever you end the code, make sure you are pulling hard at :df:. If you go all the way back to :r:, that can trigger the super.
Hope that helps a bit.