When you are wearing glasses with a strong prescription glasses, sunglasses, color contact lenses,or circular contact lenses, the device may not recognize your irises. If you are wearing those lenses, remove them before registering your irises.
And, when wearing glasses, the light reflected from glass surface can interfere the sensor, leading to failure in recognition in some cases. If it is the case, taking glasses off or adjusting the angle of sight is recommended to get it done.
If you have undergone laser-assisted vision correction surgery, such as LASIK or LASEK, the refraction of light to your eyes will change because of the change in your cornea, and the device will not recognize your irises.
Unlock the device using the pattern, PIN, or password you set when registering the irises, and then re-register your irises again before using the iris recognition feature.