Hi Hilary
I've created several forms and published to SharePoint and they have all had the same issue. They were fine on the old MOSS system. Now we are using SharePoint online it seems to be playing up and the only difference is that I have had to convert the form to work on SharePoint. SO all the submit buttons and data connections are the same. I wondered if it was something to do with being online/cloud based now as before we weren't. Strange one. Wish I was more geeky lol :) then I might have a better understanding of the background stuff.
Thanks again for your help.