custom forms - Send button is disabled

Hi

I have created a custom form and have it saved in my personal forms library.

I have attached it in a general email to several users as in insert to a
message. Showing up in the body of the email as an envelope. However when
the user opens up the envelope and fills in the questionaire the send button
is greyed out and the user is unable to send the message unless they go into
options and forward.

Any help would be greatly appreciated.

Thanks