Okay, so I've tried adding 2C 3C 623B into my H-Ryougi so my corner combo goes something like:
(j.C) 2A 5BB 2B 5C 2C 22A (TK 236B) 5BB 214C j.Cwhiff 2C 3C 623B 2C 5C 5BB j.BC j.BC airthrow/236B.
I'm going to assume that you meant 214c j.
bwhiff, because j.c will almost always hit after a 214c in the corner.
Before I even get into the second part of the question, I'd like to try to discourage you from doing this combo. You're forcing a bunch of tight links on yourself and actually limiting the amount of damage you could be doing.
For example:
When I do the combo you described from 2a, with the TKj.236b, and end with airthrow, I get 5,049 damage on V.Sion. 24 hits for 100% meter and I lose my knife.
Compare that to this:
2a, 5bb, 2b, 5c, 2c, 22a, 3c, 5bb, 214a, 2c, 5c, 5bb, j.bc, dj.bc, airthrow.
5,046 damage on V.Sion. 20 hits for no meter and I lose the knife.
Basically the same damage, and you don't use 100% meter. I think you'll find it MUCH easier to pull off as well, since the TKj.236b after 22a and the 2c after 623b aren't there.
If you've got meter to burn, try this:
2a, 5bb, 2b, 5c, 2c, 22a, 3c, 5bb, 214a, 2c, 5c, 5bb,
214c, j.bwhiff, 2c, 3c, 5c, 5bb, j.bc, dj.bc, airthrow.
Does about 100 more damage, 26 hits.
If you want a hard version:
2a, 5bb, 2b, 5c, 2c, TKj.236b, 5bb, 214a, 2c, delay, 5c, 22a, 2c, 3c, 623b, 236a236a236a, knife pickup.
5,503 damage to V.Sion, and you get to pick up the knife.
Meter burn version:
2a, 5bb, 2b, 5c, 2c, TKj.236b, 5bb, 214a, 2c, delay, 5c, 22a, 2c, 3c,
5c, 5bb, 214c, j.bwhiff, 2c, 3c, 623b, 236a236a236a, knife pickup.
5,796 damage to V.Sion.
However, I have a lot of trouble at/after the 3C 623B bit. Sometimes the combo will connect properly and I get a nice beautiful valid 24 hit combo. Other times, the enemy will be able to air-tech from the 623B so my combo ends there. Other times, the enemy will hit the floor before I finish recovering from the 623B so my 2C ends up OTG-ing them.
Is there any particular reason why this would be happening (eg. timing between 2C/3C/623B)? If so, is there any way I can fix it so that I can get a fairly consistent hitrate?
2c after a 623b in the corner is a pretty tight timing no matter what, and it kind of depends on how you did the 3c to 623b. If there are already a lot of hits in the combo like yours, you'll want 623b to go off asap after the 3c to keep them high so you have enough time to 2c. The trick to that is the input 623b asap after 3c, and make sure you let go of the b button as quickly as possible, so you don't get any charge-up time.
But even then, if you're off by even a little bit they'll get to airtech. If your 623b hit relatively late after the 3c, i.e. right before they hit the ground, then you probably won't have enough time to 2c them after the 623b and they'll groundtech. So the moral of the story is to not try it in the first place when there's a better alternative.
On a related note, that kind of late timing is very good when you have few hits in your combo, like the midscreen 623b combos. Delayed 623b after 3c, tap the b so you don't get any charge, and they should be low enough and you should still be close enough to hit a follow-up 2c.
Btw 3c is definitely untechable knockdown.