Adobe 7/8 format (level 1.6) isn't backwards compatible with older versions of adobe reader. .... my guess is some of your members haven't updated their readers.
What level of pdf is your default (the default is generally highest rather than lowest)?
To ensure full backwards compatibility you should set the PDF level to 1.2 (advanced settings) with graphics set to 96dpi (75%)
Another alternative is to get everyone to download Foxit reader which is a much smaller package (1.5Mb as opposed to 28Mb) and supports all the latest bells and whistles!
It's good even for modem users.
and will be ok for all except the hardened pro (who will have the latest version anyhow!)
http://www.foxitsoftware.com/pdf/rd_intro.php