Talk to ENT on My Mother Is Unable To Hear Sometimes

Its been a year since we noticed it. She is unable to hear sometimes. We don't think its her ears problem as when we talk she is hearing properly but sometimes she looses concentration and therefore cannot hear. When she talks on phone, she is listening fine. She is 50 years old and has been vegan since childhood. We are suspecting Vitamin b12 deficiency. Please advise me in this regard.