Are you passing a date (YYYY-MM-DD) or date time (YYYY-MM-DD HH:MM:SS)?
whereBetween is sugar for the database's BETWEEN keyword. If you need to include start and end dates, be explicit about the times i.e. YYYY-MM-DD 00:00:00 and YYYY-MM-DD 23:59:59.
The reason it includes the start date and not the end date is because of the implicit time of 00:00:00.