I Can’t confirm it because i have no copy of SSF4 where I am now, but i suppose that you lose your backcharge.
Cr. HP moves you towards your opponent (thats the reason the cr. hp-legs loops works and cs. HK - legs doesn’t).
You are then so close to your opponent, that he crosses you over while spinning around you - so you lose your backcharge and can’t get the Ultra.
I’ve noticed this problem, but I am 99% sure it is still possible.
In my experience, there is a brief window of time where the game engine will simply NOT allow you to Ultra after EX SBK, regardless whether or not you have charge.
First of all, make sure that when you execute EX SBK, you are going from down-back to up-back so that you are keeping your back-charge.
After EX SBK comes out, you can do one of two things – do the motion for Ultra 1 from Up-Back during EX SBK, so that Ultra 1 comes out right after EX SBK is done (basically a Balrog EX Headbutt > Ultra 1). OR
Continue to charge (while your opponent is in the air, falling) and do Ultra 1 as they fall within striking height of Ultra 1.
Both work, but only in specific situations. In some situations they both work, in some you have to wait, in some you have to do it instant.
c.hp x ex sbk puts chun too close, so they switch sides during ex sbk, that’s why she loses charge.
if you use pretty much any other move they will be too far out too switch sides, so combo into u1 works from that.
When I want to combo into an EX SBK (Not juggle) I often use cr. :mk: xx EX SBK since it always works and it pushes you a just enough to keep them in the corner.