Ryo's Zanretsuken always comes out as a dp for me.

[SuperGoomba64]

I seem to have this problem with any character with a forward, back, forward input. There always seems to be some overlap with another move, and that move will always come out instead of the machinegun punch. Even supers will come out as well when attempting an ex version.

I can't seem to find any help on this. If anyone has some tip or trick to prevent dp's from coming out what feels like 70% of the time that'd be a huge help.

[NYC AirLancer]

hcb-f/632146+A/C is a "shortcut" in this game for a DP. So if your 646 (forward back forward) isn't clean, it can overlap with other motions. One way to avoid this, at least if you're doing it immediately after another move (say, canceling into it off of a DP) is to go forward, back, then travel through the up motions (up-back, up, up-forward) and end on forward. This'll avoid the lower directions.

[freefall]

Yes, you need to make sure your input is precise to avoid overlaps. This is usually a problem if you're using a controller, since it's possible that you rolled the D-pad/thumb stick to a diagonal position. You should also go into Practice mode and turn on the input display to see what your inputting mistakes are, and how you can correct your motioning. Chin's 421+P requres precise motioning or you'll get 214+P instead. You should try practising the move's motioning and get a feel for it so it becomes muscle memory.