使用primefaces从数据表中单击时，为什么文件未正确下载

my p:fileDownload is not working properly in a datatable. when I click on a commandbutton in a row it downloads the file properly. but then, I when I click on the button on another one row it downloads the previous file. It is behaving differently with txt files. When I download a txt files it also downloads the previous txt file, but when I open them, it turns like the content is mixed with the previous one. Sometimes with wierd characters. (the files are stored in a database)my p:fileDownload is not working properly in a