There are several synonyms for the phrase "fail to notice," including overlook, miss, neglect, disregard, ignore, and omit. Overlooking something means to unintentionally fail to notice it, while missing something means to fail to see or hear it. Neglect and disregard both suggest a lack of attention or concern, whereas ignoring something implies a deliberate choice to not pay attention to it. Omitting something means leaving it out completely or forgetting to include it. Using these synonyms can help add variety and clarity to writing and communication, and can also help avoid repetition.