Hi @draulio, could you kindly confirm your hardware and software setup please? If indeed you are using the X2-30, this is a product belonging to Tobii Pro Business department and you should contact them regarding this @
https://www.tobiipro.com/contact/support/
In any event, failure to detect eyes in certain individuals may be linked to a number of factors such as particularly thick glasses, heavy makeup, droopy eyelids, facial piercings, etc so worth taking a look at these factors.