1. Better be sure you instruct them correctly about how to forward. You should be able to see the difference between "Re: ..." and "Fw: ..." to check.
2. Then if you're sure it's forwarded, check the source. In Outlook Express, it's in the properties of the message. It should show the detailed contents.
3. It would be interesting to see the size of the email (in bytes or kB) as send and as received.
You don't tell which email-client (like Outlook Express or Thunderbird) you have nor what these people have. The explanation could be somewhere in there.
On two recent occasions, people I sent an email message to replied that they were not able to open the attachment! However, I didn't send an attachment! Both were simple messages that originated with me. I suspect that the recipients receive as text (I send as HTML), but other people I email to have not said anything and I know that they receive as text too. I asked them to "forward" the email back to me. It's not clear to me that they knew the difference between "reply" and "forward", but in any case, no attachment came back.
What could account for this? I've done a virus scan. Has anyone heard of this?