I created a lunch order form with an e-mail submit button, many of our parents are unable to submit the form. The comments are either nothing happens or "unable to perform this operation". We have had some forms submitted so it seems to work occasionally.
It might be a good idea to ask those who can submit which version of Reader they are using; and similarly check with those who can't submit. It may lie in the version of Reader your users have...that's a good place to start. If that's not it; can you post a link to the form so we can see it?
Acrobat is probably the program I use most often and I'm learning more every day.