i ask because i hardly ever get a red eye problem with anyone. But for some reason, near every time i take a photo of my daughter she has red eye. And the unlikely times like my boys get red eye it is easily corrected in post just by clicking the red eye reduction box. which i hardly have ever had to do. My daughter, i click the box it sometimes just changes one eye, sometimes just turn them both pink. so she cant seem to really be fixed in post. which seems kid of odd to me. Doesn't seem to matter which camera i use either. Any of the three, same thing. i was wondering if eye color is a factor.
I don't know about the software problem correcting only one red eye, but red eye is the direct reflection from the eye when the flash and camera lens are very close together, at the same angle to perfectly reflect directly back.
The way you reduce red eye is to move the flash away from the lens. Or have the subject not look directly into the lens.
For example, a little compact camera has maybe 1/2 inch between the flash and the lens. Red eye is pretty much guaranteed (on such tiny cameras).
A larger DSLR camera may have two inches separation from the built in flash. This is better, but still a problem.
The DSL with a larger hot shoe speedlight may have six inches of separation. This is a lot better, but not fail safe.
A rule of thumb for direct flash is that one inch of separation for each foot of distance to the subject is enough angle difference to generally work pretty well. More separation is better. Or bounce flash is much better, very fail safe.
Off camera lighting is much better lighting anyway, and it solves red eye too.