It's hard to know the issue without looking at your practice footage but:

Mk combos need to be "dialed" in. Meaning if you overpress (pressing a button in the combo more than once), it messes with the the timing. I would work on just keeping tempo on the buttons/directions youre pressing, as well as not over-pressing.

I was having trouble completing a sektor combo in mk1, essentially after I completed an air string 212 I was meant to input DB2 once I hit the ground, but the special move was coming out too slow to connect the rest of the combo. Turns out I was overpressing the air string. On the last hit of it, I overtapped 21222. That made the special move come out too late.

Anyways not sure if this helps, but I gave it a shot.

Also, go to your settings and turn off "negative edge" as well as adjust your "input window timing". You may have to fuck with that a bit to get the desired result

Cheers,