Agree’ing with Xaaz, here.
I never struggled with JabShortHB in any standalone non-EX combo but I had a massive lack of ability to EX Loop for a similar reason to your problem.
Until that point I never understood links/cancels (I wouldn’t even be able to tell you what they are ‘exactly’ even now) but I took special notice of what worked/what didn’t work and eventually half sussed it out as to why I couldn’t connect more than one or two loops (if lucky) and especially not headbutt after any of them and it’s because of the rythm I was trying to punch out the moves.
My initial idea was to mash jabs after an Upper, hoping and praying I’d connect one after an Upper - Absolutely no luck. Paced it down and eventually realised you have to get it right at the end of the Upper. From there I found that I could get UP>Jab>Short, but no Headbutt, and it was the same deal - I wasn’t waiting for my Jab to fully end before pressing a Short. Watched many movies of EX Loops and realised how much slower it seemed to come out when people where looping specifically (Not so much non-loop JabShortHB as you can pretty much throw them out very quickly in comparison to when you’re trying to loop them).
So once I realised I had to Jab at the pinnacle of the EX Upper, and Short at a slight delay, giving time for the Jab to end, the Headbutt comes out 100%.
Pace yourself downward - Even from a simple Jab, your opponent has a noticeable few frames of hitstun for you to get the next move you want out so don’t go bashing that short the moment you notice you’ve hit with the jabs - All about timing and getting a rythm and you’ll find that once you get it, it never leaves you.
I still to this very day have NOT looked at Frame Data, researched into links/cancels or any fancy stuff and yet I’m merrily quad-looping and performing all sorts of combos relatively consistantly just from trial and error.
As simple as JabShortHeadbutt is, it’s the window to so many other combos in Boxers arsenal and quite a few (if not mostly all other) variations of it, regardless of the moves used, are very similar.
Once you get this down, a LOT of things will come so much easier to you so I wish you luck with it. Pace it down, experiment with it and once you get it, practice it over and over and over - It’ll burn into your mind and you’ll be doing them on reaction in no time.
One of the latest fun things I’ve been messing with are completely swapping the normals. Jumping Fierce Short CJab Short CJab Headbutt is completely mundane but funny to see - As said, once you get the basics down you’ll have fun experimenting with all it’s variations and uses (if any).