Stun Juggle Combo Char Specific?

In training room I can get multiple reps of ex ducking straights on the bigger hitbox chars, but when I set it to Ryu I can only seem to get ducking straight into ex ducking upper. I was wondering if this was a char specific thing or if I’m just screwing up the timing. Also any tips on visual cues or anything that helps/helped you guys get the stun combo down would be much appresh’d.

it’s char specific. I’m pretty sure it’s detailed in tut.