It looks like we have an answer (As of episode 20).
The reason her eye didn't regenerate may be due to something along the lines of life fibers not being able to perfectly repair everything. Or maybe if you cut it the right way with the scissor blade a body part won't regenerate.
There's also the possibility that her eye is fine and she's just covering it up for some reason.
Oh man, that'd definitely be interesting; Ryuuko could probably use the scissor blade to actually kill Ragyo and maybe even free her own body from the life fibers, then.
Still, the fact that Ragyo/Nui haven't mentioned (I doubt they haven't noticed) the fact that Nui's eye hasn't regenerated is a bit weird. Either they're trying to keep it under wraps, or, like you said, she's lying about the eye being gone to trick Ryuuko, or something along those lines.
Hmm, maybe the fact that it hasn't regenerated may be due to a unique factor of the Purple Scissor Blade that Isshin used to stab her; maybe it prevents Life Fibers infused in a human body from regenerating? Either that or her eye is fine (or has gained some sort of special property that requires her to cover it up). Although her reaction when Isshin took her eye (screaming and running away) is very different from when Ryuko stabbed her (when she faked being wounded before healing) indicating that she was actually injured, at least enough to ditch her mission and flee with the Scissor Blade.
I believe when Isshin stabbed her eye, he did so with the Rending Scissors, whereas Ryuko only stabbed her with the Scissor Blade, which mightn't be powerful enough. Then again, I don't remember if Isshin used the Rending Scissors or just one blade.
He only used the purple blade. The red blade was still impaling him.
It might be possible that the properties of the two blades differ in some way.
That's also true. After all, the red half can turn into a greatsword, whereas in the latest episode, Nui used hers as a scythe.
I dont know how many fucking Times i have to write this - BUT THE SCISSOR WAS RED WHEN ISSHIN USED IT.
She only changed the Color of said Blade since... (What a Surprise) , Nui likes Purple.
Really, how can people be so colorblind?
True, it was originally red. But it's still possible that the two blades had/have different properties; as stated previously, one turns into a longsword while the other turns into a double-bladed scythe.
Guys, the life fibers haven't really been shown to regenerate, just stick things back together. Raygo didn't sprout a new hand or head when they were cut, she stuck the old one back on. I seriously doubt the blade itself has anything to do with it, Nui probably just doesn't have her old eye anymore.
Also, I would like to point out that the one scissor we know a lot about has been turned into different forms. Mostly just the tiny scissor, but still, we don't know if the scythe form is unique to Nui's blade or just Nui's preferred form to use it in. While it's possible that they can do different things, the fact that they've been shown to do things that seem within the limits of or possible for each of them and the fact that they were originally one item suggests they're the same, the only differences coming from training and usage.
I think as to why it had cut her eye was because the scissors were only JUST pulled apart before she got her eye cut. When Ryuuko uses both blades to cut Nui's arms off, thats when they actually get severed. The blades have to be together in order to sever life fibers. At least thats what i think.