Honestly, I would ignore the "Combo Throw" prompts. They never help me or seem accurate for the Izunas.
I find that in the case of the Otoshi, after doing the first two inputs I generally make two full 360 rotations before hitting the T button.
Shoho's weird. I actually mastered the Shoho back before I ever used the throw or hold variants, and have generally found it the easiest to perform. Recently, however, this hasn't been the case. I think I'm just going through a bad spell, but sometimes I'll drop it, and it feels like I should have gotten it (of course, this is online). Then I'll do the same thing ten times in practice, and it'll work every time. It's really beginning to irritate me because I never had this problem before, and now that it's here, I don't know how to fix it. Hopefully, just giving it some time will set me back on track.