Gmail Spam Filter Not Working can be caused by a number of factors. Many people make the mistake of clicking 'Not Spam' on emails that are spam in actuality while looking through their spam folder. All of the spam messages end up in your inbox as a result of this unintentional click. Alternatively, if you click any link in a spam email, you can be sent to a site that looks legitimate but is actually a place where your information is stolen. This can be another reason behind this error.