I'm unable to filter google spreadsheet without creating a new filter

I'm relatively new to Google Spreadsheets and I've some issues using it. I've got a spreadsheet where I'm unable to filter its content without creating a new filter view. Normally I can select the top header row, press the filter button and then triangle appears on top of each column letting me to apply a filter to any column without any hassle. In this spreadsheet I must create a new filter view in order to be able to filter its content.