If Excel filter is not working after certain row, for merged cells, on large files or on protected sheet, then see this post.

How to Tell If a Worksheet Is Protected in Microsoft Excel 2003. Protecting a worksheet in Excel 2003 prevents others users from editing it. For example, if you have tracked and charted your ...

Using Microsoft Excel 2007, you can protect worksheets in order to keep other users from making changes. Unfortunately, this protection applies to everything on the worksheet, including groups.