This problem may all come down to your keyboard. It's called "Rollover". You keyboard wont allow you to press certain combinations of keys all at once.
You can edit the character's CMD file to make it so the move only requires 2 keys pressed at one time.
This is a small example of how to make a 3 button special move into 3 different commands
[Command]
name = "tatsunoko"
command = ~B, DB, D, DF, F, a+b+c
[Command]
name = "tatsunoko"
command = ~B, DB, D, DF, F, a+b
[Command]
name = "tatsunoko"
command = ~B, DB, D, DF, F, b+c
[Command]
name = "tatsunoko"
command = ~B, DB, D, DF, F, a+c
In the long run the special can be performed easier with just the two button presses. But it helps with people playing on keyboards.