i would have to look at exactly what you were doing. you might be pulling from different group lists or something and some have a bad email address.
but i hope that you wont take offense if i say a possibility is that they do actually get the email but either dont notice or delete it but they cant say they never got it if you tell them you copied yourself.
i havent tried yet
what email program are you using?
do they get the email but not the attachment ?
i wonder if you are sending them as a link rather than the actual spreadsheet and they may not have rights to the attachment unless you are on the email and then it attaches it because it logs you in for it. or something similar.
make sure that anybody that you are sending to has rights to access those documents on your google.
try downloading it as an excel sheet and then emailing it still through gmail
if nothing else do it as a test please so that we can pinpoint exactly where the problem is.