One of the most common problem with filter function is that it stops working beyond a blank row. Filter will not include cells beyond the first blank.
To avoid this issue, select the range before applying the filter function. Please refer to the following excel spreadsheet for example.
1. Before applying filter on column C, either select the entire column C or the data that needs to be filtered.
2. Select Filter option under Data tab. Now all items appear in the filtered list as well as filter checkbox list.
Similarly you may select multiple columns or a range of cells before applying the filter.
External links for other issues that might prevent filter function to work properly.
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