From Longman Dictionary of Contemporary EnglishI fail to see/understandI fail to see/understandformalUNDERSTAND used to show that you are annoyed by something that you do not accept or understand I fail to see why you find it so amusing. → fail
Examples from the Corpus
I fail to see/understand• However I fail to see firstly what you are trying to prove with your figures.• How presumptuous my claims to knowledge based only on what I see, leaving out what I fail to see.• Last year? I failed to see the connection.• I fail to see the humor in this situation. 