Are you saving the XL file as a .xlsx format which is the default for Excel 2007 onwards.
Some of your members may be running earlier versions which can not open these files. There is a Microsoft conversion add on but the easiest way to cure this is for you to save the files in .xls format which you can select in the lower box when saving the file. You can also select .xls as the default format for all new XL files by selecting this in the Options. All versions of XL can open .xls files so you lose nothing unless you are using some of the more special faciliies of an xlsx format.